diff --git a/.github/workflows/code-style.yml b/.github/workflows/code-style.yml
new file mode 100644
index 0000000..728fa56
--- /dev/null
+++ b/.github/workflows/code-style.yml
@@ -0,0 +1,37 @@
+name: Code style
+
+on:
+  push:
+  pull_request:
+
+jobs:
+
+  check-code-style:
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ['8.2']
+
+    name: "Check code style | PHP ${{ matrix.php }}"
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Install PHP with latest composer
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          ini-values: error_reporting=-1, display_errors=On, log_errors_max_len=0
+          coverage: none
+          tools: none
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: "Install Composer dependencies"
+        uses: "ramsey/composer-install@v2"
+
+      - name: "Check code style"
+        run: composer run codestyle -- --dry-run
diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index e159e9f..6de33d4 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -11,7 +11,13 @@
 
 return (new PhpCsFixer\Config())
     ->setRules([
-        '@PSR12' => true,
+        '@PER-CS2.0' => true,
+        '@PER-CS2.0:risky' => true,
+        '@PHP74Migration' => true,
+        '@PHP74Migration:risky' => true,
+        '@PHPUnit84Migration:risky' => true,
+        'no_alias_functions' => true,
     ])
     ->setFinder($finder)
+    ->setRiskyAllowed(true)
 ;
diff --git a/README.md b/README.md
index 1325e1a..8954472 100644
--- a/README.md
+++ b/README.md
@@ -118,7 +118,7 @@ Please see [CHANGELOG](CHANGELOG.md) for more information what has changed recen
 
 ## :wrench: Contributing
 
-Please feel free to fork and sending Pull Requests. This project follows [Semantic Versioning 2](http://semver.org) and [PSR-2](http://www.php-fig.org/psr/psr-2/).
+Please feel free to fork and sending Pull Requests. This project follows [Semantic Versioning 2](http://semver.org) and [PER-CS2.0](https://www.php-fig.org/per/coding-style/).
 
 This projects comes with a `docker-compose.yml` where all tools for development are available.
 
diff --git a/composer.json b/composer.json
index 99c926e..82738e1 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
         "php": "^7.4 || ~8.0.0 || ~8.1.0 || ~8.2.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^3.2",
+        "friendsofphp/php-cs-fixer": "^3.35",
         "phpstan/phpstan": "^1.10",
         "phpunit/phpunit": "^9 || ^10"
     },
@@ -31,6 +31,7 @@
         }
     },
     "scripts": {
+        "codestyle": "php-cs-fixer fix --verbose --diff",
         "coverage": "phpunit --coverage-html=\".phpunit.cache/code-coverage\"",
         "phpstan": "phpstan analyze --memory-limit 512M --configuration .phpstan.neon",
         "phpunit": "phpunit",
diff --git a/src/Accessable.php b/src/Accessable.php
index 7fe7b13..ec46ab1 100644
--- a/src/Accessable.php
+++ b/src/Accessable.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Element.php b/src/Element.php
index 966ebbe..2951a2e 100644
--- a/src/Element.php
+++ b/src/Element.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Exception/AccessException.php b/src/Exception/AccessException.php
index 12e3ea3..9edf22e 100644
--- a/src/Exception/AccessException.php
+++ b/src/Exception/AccessException.php
@@ -1,11 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Art4\JsonApiClient\Exception;
 
-class AccessException extends \RuntimeException implements Exception, \Throwable
-{
-}
+class AccessException extends \RuntimeException implements Exception, \Throwable {}
diff --git a/src/Exception/Exception.php b/src/Exception/Exception.php
index ff5664b..0e88f54 100644
--- a/src/Exception/Exception.php
+++ b/src/Exception/Exception.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -10,6 +12,4 @@
  * @deprecated `\Art4\JsonApiClient\Exception\Exception` will extend `\Throwable` in v2.0. Do the same in your implementation now to avoid errors.
  */
 // interface Exception extends \Throwable
-interface Exception
-{
-}
+interface Exception {}
diff --git a/src/Exception/FactoryException.php b/src/Exception/FactoryException.php
index f9c97c1..3b8ffbb 100644
--- a/src/Exception/FactoryException.php
+++ b/src/Exception/FactoryException.php
@@ -1,11 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Art4\JsonApiClient\Exception;
 
-class FactoryException extends \Exception implements Exception, \Throwable
-{
-}
+class FactoryException extends \Exception implements Exception, \Throwable {}
diff --git a/src/Exception/InputException.php b/src/Exception/InputException.php
index 928584c..b59fe12 100644
--- a/src/Exception/InputException.php
+++ b/src/Exception/InputException.php
@@ -1,11 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Art4\JsonApiClient\Exception;
 
-class InputException extends \Exception implements Exception, \Throwable
-{
-}
+class InputException extends \Exception implements Exception, \Throwable {}
diff --git a/src/Exception/ValidationException.php b/src/Exception/ValidationException.php
index e8575d2..771e20a 100644
--- a/src/Exception/ValidationException.php
+++ b/src/Exception/ValidationException.php
@@ -1,11 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Art4\JsonApiClient\Exception;
 
-class ValidationException extends \InvalidArgumentException implements Exception, \Throwable
-{
-}
+class ValidationException extends \InvalidArgumentException implements Exception, \Throwable {}
diff --git a/src/Factory.php b/src/Factory.php
index 84f1613..7dd1ed9 100644
--- a/src/Factory.php
+++ b/src/Factory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Helper/AbstractElement.php b/src/Helper/AbstractElement.php
index 4010412..f15a62c 100644
--- a/src/Helper/AbstractElement.php
+++ b/src/Helper/AbstractElement.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Helper/AccessKey.php b/src/Helper/AccessKey.php
index 0205563..a57558f 100644
--- a/src/Helper/AccessKey.php
+++ b/src/Helper/AccessKey.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Helper/AccessableTrait.php b/src/Helper/AccessableTrait.php
index a24e633..81eb8bf 100644
--- a/src/Helper/AccessableTrait.php
+++ b/src/Helper/AccessableTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Helper/Parser.php b/src/Helper/Parser.php
index f903515..1485f59 100644
--- a/src/Helper/Parser.php
+++ b/src/Helper/Parser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Helper/RootAccessable.php b/src/Helper/RootAccessable.php
index f7fa3ea..78c46ba 100644
--- a/src/Helper/RootAccessable.php
+++ b/src/Helper/RootAccessable.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Input/Input.php b/src/Input/Input.php
index 01a0ecf..457a534 100644
--- a/src/Input/Input.php
+++ b/src/Input/Input.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Input/RequestInput.php b/src/Input/RequestInput.php
index 0623725..1b3ab1f 100644
--- a/src/Input/RequestInput.php
+++ b/src/Input/RequestInput.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -9,6 +11,4 @@
 /**
  * Request Input Interface
  */
-interface RequestInput
-{
-}
+interface RequestInput {}
diff --git a/src/Input/RequestStringInput.php b/src/Input/RequestStringInput.php
index 36973b8..322c62b 100644
--- a/src/Input/RequestStringInput.php
+++ b/src/Input/RequestStringInput.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Input/ResponseStringInput.php b/src/Input/ResponseStringInput.php
index ae47337..10a9b30 100644
--- a/src/Input/ResponseStringInput.php
+++ b/src/Input/ResponseStringInput.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Input/StringInputTrait.php b/src/Input/StringInputTrait.php
index ffbfd22..a343026 100644
--- a/src/Input/StringInputTrait.php
+++ b/src/Input/StringInputTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -49,7 +51,7 @@ final protected function decodeJson(string $jsonString)
             \JSON_ERROR_STATE_MISMATCH => 'JSON_ERROR_STATE_MISMATCH - Underflow or the modes mismatch',
             \JSON_ERROR_CTRL_CHAR => 'JSON_ERROR_CTRL_CHAR - Unexpected control character found',
             \JSON_ERROR_SYNTAX => 'JSON_ERROR_SYNTAX - Syntax error, malformed JSON',
-            \JSON_ERROR_UTF8 => 'JSON_ERROR_UTF8 - Malformed UTF-8 characters, possibly incorrectly encoded'
+            \JSON_ERROR_UTF8 => 'JSON_ERROR_UTF8 - Malformed UTF-8 characters, possibly incorrectly encoded',
         ];
 
         // use JSON_BIGINT_AS_STRING
diff --git a/src/Manager.php b/src/Manager.php
index 92de711..b22dde4 100644
--- a/src/Manager.php
+++ b/src/Manager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Manager/ErrorAbortManager.php b/src/Manager/ErrorAbortManager.php
index 4451bd3..996d3c5 100644
--- a/src/Manager/ErrorAbortManager.php
+++ b/src/Manager/ErrorAbortManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Serializer/ArraySerializer.php b/src/Serializer/ArraySerializer.php
index 7ad829c..42d1491 100644
--- a/src/Serializer/ArraySerializer.php
+++ b/src/Serializer/ArraySerializer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/Serializer/Serializer.php b/src/Serializer/Serializer.php
index 54b5269..823ebe1 100644
--- a/src/Serializer/Serializer.php
+++ b/src/Serializer/Serializer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Attributes.php b/src/V1/Attributes.php
index f3e81f4..918dcf3 100644
--- a/src/V1/Attributes.php
+++ b/src/V1/Attributes.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Document.php b/src/V1/Document.php
index 00af358..ff3194d 100644
--- a/src/V1/Document.php
+++ b/src/V1/Document.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/DocumentLink.php b/src/V1/DocumentLink.php
index 8fe2af4..30ab3c3 100644
--- a/src/V1/DocumentLink.php
+++ b/src/V1/DocumentLink.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Error.php b/src/V1/Error.php
index 28f1c5b..69c2f7b 100644
--- a/src/V1/Error.php
+++ b/src/V1/Error.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ErrorCollection.php b/src/V1/ErrorCollection.php
index eab6b48..42def2f 100644
--- a/src/V1/ErrorCollection.php
+++ b/src/V1/ErrorCollection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ErrorLink.php b/src/V1/ErrorLink.php
index 13b5d62..112d09b 100644
--- a/src/V1/ErrorLink.php
+++ b/src/V1/ErrorLink.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ErrorSource.php b/src/V1/ErrorSource.php
index 87a8b57..3c70b99 100644
--- a/src/V1/ErrorSource.php
+++ b/src/V1/ErrorSource.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Factory.php b/src/V1/Factory.php
index f1bab75..78dc1dc 100644
--- a/src/V1/Factory.php
+++ b/src/V1/Factory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Jsonapi.php b/src/V1/Jsonapi.php
index 66f91d4..db56bef 100644
--- a/src/V1/Jsonapi.php
+++ b/src/V1/Jsonapi.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Link.php b/src/V1/Link.php
index 147899a..1c07bae 100644
--- a/src/V1/Link.php
+++ b/src/V1/Link.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Meta.php b/src/V1/Meta.php
index d03826d..5836bab 100644
--- a/src/V1/Meta.php
+++ b/src/V1/Meta.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/Relationship.php b/src/V1/Relationship.php
index 0e53f75..5a842cf 100644
--- a/src/V1/Relationship.php
+++ b/src/V1/Relationship.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/RelationshipCollection.php b/src/V1/RelationshipCollection.php
index 0d4e4e9..49680f0 100644
--- a/src/V1/RelationshipCollection.php
+++ b/src/V1/RelationshipCollection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/RelationshipLink.php b/src/V1/RelationshipLink.php
index 11e2f85..84b1c59 100644
--- a/src/V1/RelationshipLink.php
+++ b/src/V1/RelationshipLink.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceCollection.php b/src/V1/ResourceCollection.php
index e29555d..4577e2b 100644
--- a/src/V1/ResourceCollection.php
+++ b/src/V1/ResourceCollection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceIdentifier.php b/src/V1/ResourceIdentifier.php
index 037cb57..26e6b2c 100644
--- a/src/V1/ResourceIdentifier.php
+++ b/src/V1/ResourceIdentifier.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceIdentifierCollection.php b/src/V1/ResourceIdentifierCollection.php
index 03aa1d6..a887070 100644
--- a/src/V1/ResourceIdentifierCollection.php
+++ b/src/V1/ResourceIdentifierCollection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceItem.php b/src/V1/ResourceItem.php
index 32fae43..caae85a 100644
--- a/src/V1/ResourceItem.php
+++ b/src/V1/ResourceItem.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceItemLink.php b/src/V1/ResourceItemLink.php
index dbecb4f..69ce850 100644
--- a/src/V1/ResourceItemLink.php
+++ b/src/V1/ResourceItemLink.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/src/V1/ResourceNull.php b/src/V1/ResourceNull.php
index 792c237..c7df660 100644
--- a/src/V1/ResourceNull.php
+++ b/src/V1/ResourceNull.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -62,7 +64,7 @@ public function getKeys()
      *
      * @param string $key The key of the value
      */
-    public function get($key)
+    public function get($key): void
     {
         throw new AccessException('A ResourceNull has no values.');
     }
diff --git a/tests/BC/AccessableTest.php b/tests/BC/AccessableTest.php
index 4a4c67b..7ee74b8 100644
--- a/tests/BC/AccessableTest.php
+++ b/tests/BC/AccessableTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,7 +18,7 @@ class AccessableTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForAccessableInterface()
+    public function testBcForAccessableInterface(): void
     {
         $class = new class () implements Accessable {
             /**
diff --git a/tests/BC/ElementTest.php b/tests/BC/ElementTest.php
index 5644652..73c28e3 100644
--- a/tests/BC/ElementTest.php
+++ b/tests/BC/ElementTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -18,7 +20,7 @@ class ElementTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForElementInterface()
+    public function testBcForElementInterface(): void
     {
         $data = null;
         $manager = $this->createMock(Manager::class);
@@ -30,9 +32,7 @@ public function testBcForElementInterface()
              *
              * @param mixed $data The data for this Element
              */
-            public function __construct($data, Manager $manager, Accessable $parent)
-            {
-            }
+            public function __construct($data, Manager $manager, Accessable $parent) {}
         };
 
         $this->assertInstanceOf(Element::class, $class);
diff --git a/tests/BC/Exception/ExceptionTest.php b/tests/BC/Exception/ExceptionTest.php
index 4ee7f01..c84d828 100644
--- a/tests/BC/Exception/ExceptionTest.php
+++ b/tests/BC/Exception/ExceptionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,10 +18,9 @@ class ExceptionTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForExceptionInterface()
+    public function testBcForExceptionInterface(): void
     {
-        $class = new class () implements Exception {
-        };
+        $class = new class () implements Exception {};
 
         $this->assertInstanceOf(Exception::class, $class);
     }
diff --git a/tests/BC/FactoryTest.php b/tests/BC/FactoryTest.php
index c6fe5a6..beb1ecd 100644
--- a/tests/BC/FactoryTest.php
+++ b/tests/BC/FactoryTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,7 +18,7 @@ class FactoryTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForFactoryInterface()
+    public function testBcForFactoryInterface(): void
     {
         $class = new class () implements Factory {
             /**
diff --git a/tests/BC/Input/InputTest.php b/tests/BC/Input/InputTest.php
index f62a92e..1e0d0c7 100644
--- a/tests/BC/Input/InputTest.php
+++ b/tests/BC/Input/InputTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,7 +18,7 @@ class InputTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForInputInterface()
+    public function testBcForInputInterface(): void
     {
         $class = new class () implements Input {
             /**
diff --git a/tests/BC/Input/RequestInputTest.php b/tests/BC/Input/RequestInputTest.php
index f96c07a..c52b939 100644
--- a/tests/BC/Input/RequestInputTest.php
+++ b/tests/BC/Input/RequestInputTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,10 +18,9 @@ class RequestInputTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForRequestInputInterface()
+    public function testBcForRequestInputInterface(): void
     {
-        $class = new class () implements RequestInput {
-        };
+        $class = new class () implements RequestInput {};
 
         $this->assertInstanceOf(RequestInput::class, $class);
     }
diff --git a/tests/BC/ManagerTest.php b/tests/BC/ManagerTest.php
index 8af9586..34005a8 100644
--- a/tests/BC/ManagerTest.php
+++ b/tests/BC/ManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -17,7 +19,7 @@ class ManagerTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForManagerInterface()
+    public function testBcForManagerInterface(): void
     {
         $class = new class () implements Manager {
             /**
diff --git a/tests/BC/Serializer/SerializerTest.php b/tests/BC/Serializer/SerializerTest.php
index 99b65e6..6ce99fd 100644
--- a/tests/BC/Serializer/SerializerTest.php
+++ b/tests/BC/Serializer/SerializerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -17,7 +19,7 @@ class SerializerTest extends TestCase
      * DO NOT CHANGE THIS!
      * Changes are only allowed by increasing the major version number.
      */
-    public function testBcForSerializerInterface()
+    public function testBcForSerializerInterface(): void
     {
         $class = new class () implements Serializer {
             /**
diff --git a/tests/Fixtures/AccessableElement.php b/tests/Fixtures/AccessableElement.php
index f1a92de..79e13fa 100644
--- a/tests/Fixtures/AccessableElement.php
+++ b/tests/Fixtures/AccessableElement.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -12,6 +14,4 @@
 /**
  * AccessableElement Interface
  */
-interface AccessableElement extends Accessable, Element
-{
-}
+interface AccessableElement extends Accessable, Element {}
diff --git a/tests/Fixtures/Factory.php b/tests/Fixtures/Factory.php
index f1c8d6e..b6d916c 100644
--- a/tests/Fixtures/Factory.php
+++ b/tests/Fixtures/Factory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/tests/Fixtures/HelperTrait.php b/tests/Fixtures/HelperTrait.php
index 74c36b2..94cc0d1 100644
--- a/tests/Fixtures/HelperTrait.php
+++ b/tests/Fixtures/HelperTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -160,7 +162,7 @@ public function buildManagerMock()
     /**
      * Builds a Manager Mock and set it into the TestCase
      */
-    public function setUpManagerMock()
+    public function setUpManagerMock(): void
     {
         // Mock factory
         $factory = new V1Factory($this);
diff --git a/tests/Fixtures/V1Factory.php b/tests/Fixtures/V1Factory.php
index e931e3f..282ea05 100644
--- a/tests/Fixtures/V1Factory.php
+++ b/tests/Fixtures/V1Factory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
diff --git a/tests/Functional/DotNotationTest.php b/tests/Functional/DotNotationTest.php
index 5d867c7..bb0a1f2 100644
--- a/tests/Functional/DotNotationTest.php
+++ b/tests/Functional/DotNotationTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -21,7 +23,7 @@ class DotNotationTest extends TestCase
     /**
      * @test
      */
-    public function testCompleteResourceObjectWithMultipleRelationships()
+    public function testCompleteResourceObjectWithMultipleRelationships(): void
     {
         $manager = new ErrorAbortManager(new Factory());
 
@@ -190,7 +192,7 @@ public function testCompleteResourceObjectWithMultipleRelationships()
     /**
      * @test
      */
-    public function testGetNotExistentValueThrowsException()
+    public function testGetNotExistentValueThrowsException(): void
     {
         $manager = new ErrorAbortManager(new Factory());
 
diff --git a/tests/Functional/ErrorParsingTest.php b/tests/Functional/ErrorParsingTest.php
index 0aea92c..ce30a29 100644
--- a/tests/Functional/ErrorParsingTest.php
+++ b/tests/Functional/ErrorParsingTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -17,7 +19,7 @@ class ErrorParsingTest extends TestCase
     /**
      * @test
      */
-    public function testParseErrors()
+    public function testParseErrors(): void
     {
         $string = $this->getJsonString('09_errors.json');
         $document = Parser::parseResponseString($string);
@@ -90,7 +92,7 @@ public function testParseErrors()
     /**
      * @test
      */
-    public function testParseErrorWithLinks()
+    public function testParseErrorWithLinks(): void
     {
         $string = $this->getJsonString('10_error_with_links.json');
         $document = Parser::parseResponseString($string);
diff --git a/tests/Functional/ParsingTest.php b/tests/Functional/ParsingTest.php
index 43b9aad..40565e1 100644
--- a/tests/Functional/ParsingTest.php
+++ b/tests/Functional/ParsingTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -40,7 +42,7 @@ public static function createParserProvider(): array
      *
      * @param mixed $parser
      */
-    public function testParseSimpleResourceWithDifferentParser($parser)
+    public function testParseSimpleResourceWithDifferentParser($parser): void
     {
         $string = $this->getJsonString('01_simple_resource.json');
         $document = $parser($string);
@@ -69,7 +71,7 @@ public function testParseSimpleResourceWithDifferentParser($parser)
     /**
      * @test
      */
-    public function testParseSimpleResourceIdentifier()
+    public function testParseSimpleResourceIdentifier(): void
     {
         $string = $this->getJsonString('02_simple_resource_identifier.json');
         $document = Parser::parseResponseString($string);
@@ -93,7 +95,7 @@ public function testParseSimpleResourceIdentifier()
     /**
      * @test
      */
-    public function testParseResourceObject()
+    public function testParseResourceObject(): void
     {
         $string = $this->getJsonString('03_resource_object.json');
         $document = Parser::parseResponseString($string);
@@ -161,7 +163,7 @@ public function testParseResourceObject()
     /**
      * @test
      */
-    public function testParseCompleteResourceObjectWithMultipleRelationships()
+    public function testParseCompleteResourceObjectWithMultipleRelationships(): void
     {
         $string = $this->getJsonString('04_complete_document_with_multiple_relationships.json');
         $document = Parser::parseResponseString($string);
@@ -336,7 +338,7 @@ public function testParseCompleteResourceObjectWithMultipleRelationships()
     /**
      * @test
      */
-    public function testParsePaginationExample()
+    public function testParsePaginationExample(): void
     {
         $string = $this->getJsonString('06_pagination_example.json');
         $document = Parser::parseResponseString($string);
@@ -390,7 +392,7 @@ public function testParsePaginationExample()
     /**
      * @test
      */
-    public function testParseRelationshipExample()
+    public function testParseRelationshipExample(): void
     {
         $string = $this->getJsonString('07_relationship_example_without_data.json');
         $document = Parser::parseResponseString($string);
@@ -456,7 +458,7 @@ public function testParseRelationshipExample()
     /**
      * @test
      */
-    public function testParseObjectLinksExample()
+    public function testParseObjectLinksExample(): void
     {
         $string = $this->getJsonString('08_object_links.json');
         $document = Parser::parseResponseString($string);
@@ -499,7 +501,7 @@ public function testParseObjectLinksExample()
     /**
      * @test
      */
-    public function testParseResourceIdentifierWithMeta()
+    public function testParseResourceIdentifierWithMeta(): void
     {
         $string = $this->getJsonString('11_resource_identifier_with_meta.json');
         $document = Parser::parseResponseString($string);
@@ -525,7 +527,7 @@ public function testParseResourceIdentifierWithMeta()
     /**
      * @test
      */
-    public function testParseNullResource()
+    public function testParseNullResource(): void
     {
         $string = $this->getJsonString('12_null_resource.json');
         $document = Parser::parseResponseString($string);
@@ -546,7 +548,7 @@ public function testParseNullResource()
     /**
      * @test
      */
-    public function testParseResourceIdentifierCollectionWithMeta()
+    public function testParseResourceIdentifierCollectionWithMeta(): void
     {
         $string = $this->getJsonString('13_collection_with_resource_identifier_with_meta.json');
         $document = Parser::parseResponseString($string);
@@ -583,7 +585,7 @@ public function testParseResourceIdentifierCollectionWithMeta()
     /**
      * @test
      */
-    public function testParseCreateResourceWithoutId()
+    public function testParseCreateResourceWithoutId(): void
     {
         $string = $this->getJsonString('14_create_resource_without_id.json');
         $document = Parser::parseRequestString($string);
@@ -595,7 +597,7 @@ public function testParseCreateResourceWithoutId()
     /**
      * @test
      */
-    public function testParseCreateShortResourceWithoutId()
+    public function testParseCreateShortResourceWithoutId(): void
     {
         $string = $this->getJsonString('15_create_resource_without_id.json');
         $document = Parser::parseRequestString($string);
@@ -607,7 +609,7 @@ public function testParseCreateShortResourceWithoutId()
     /**
      * @test
      */
-    public function testExceptionIfIdIsNotString()
+    public function testExceptionIfIdIsNotString(): void
     {
         $this->expectException(\Art4\JsonApiClient\Exception\ValidationException::class);
         $string = $this->getJsonString('16_type_and_id_as_integer.json');
@@ -617,7 +619,7 @@ public function testExceptionIfIdIsNotString()
     /**
      * @test
      */
-    public function testParseLinksInRelationshipsCorrectly()
+    public function testParseLinksInRelationshipsCorrectly(): void
     {
         $string = $this->getJsonString('17_relationship_links.json');
         $document = Parser::parseResponseString($string);
diff --git a/tests/Functional/SerializerTest.php b/tests/Functional/SerializerTest.php
index f4ddb2d..f96c043 100644
--- a/tests/Functional/SerializerTest.php
+++ b/tests/Functional/SerializerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -36,7 +38,7 @@ public static function jsonapiDataProvider(): array
 
             // Ignore files with errors
             if (in_array($filename, [
-                '16_type_and_id_as_integer.json'
+                '16_type_and_id_as_integer.json',
             ])) {
                 continue;
             }
@@ -44,7 +46,7 @@ public static function jsonapiDataProvider(): array
             $files[] = [
                 $filename,
                 [
-                    'is_request' => in_array($filename, $requestFiles)
+                    'is_request' => in_array($filename, $requestFiles),
                 ],
             ];
         }
@@ -58,7 +60,7 @@ public static function jsonapiDataProvider(): array
      *
      * @param mixed $filename
      */
-    public function parseJsonapiDataWithErrorAbortManager($filename, array $meta)
+    public function parseJsonapiDataWithErrorAbortManager($filename, array $meta): void
     {
         $manager = new ErrorAbortManager(new Factory());
 
diff --git a/tests/Unit/Helper/ParserTest.php b/tests/Unit/Helper/ParserTest.php
index 084bbff..4642729 100644
--- a/tests/Unit/Helper/ParserTest.php
+++ b/tests/Unit/Helper/ParserTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -16,7 +18,7 @@ class ParserTest extends TestCase
     /**
      * @test parseResponseBody() with valid JSON API returns Document Object
      */
-    public function testParseResponseBodyWithValidJsonapiReturnsDocument()
+    public function testParseResponseBodyWithValidJsonapiReturnsDocument(): void
     {
         $jsonapi = '{"meta":{}}';
 
@@ -26,7 +28,7 @@ public function testParseResponseBodyWithValidJsonapiReturnsDocument()
     /**
      * @test parseResponseBody throw Exception if input is invalid jsonapi
      */
-    public function testParseResponseBodyWithInvalidJsonapiThrowsException()
+    public function testParseResponseBodyWithInvalidJsonapiThrowsException(): void
     {
         $invalid_jsonapi = '["This is valid JSON", "but invalid JSON API"]';
 
@@ -41,7 +43,7 @@ public function testParseResponseBodyWithInvalidJsonapiThrowsException()
     /**
      * JSON API documents are defined in JavaScript Object Notation (JSON) [RFC4627].
      */
-    public function testParseResponseBodyWithInvalidJsonThrowsException()
+    public function testParseResponseBodyWithInvalidJsonThrowsException(): void
     {
         $invalid_json = 'invalid_json_string';
 
@@ -56,7 +58,7 @@ public function testParseResponseBodyWithInvalidJsonThrowsException()
     /**
      * @test isValidResponseBody() with valid JSON API returns true
      */
-    public function testIsValidResponseBodyWithValidJsonapi()
+    public function testIsValidResponseBodyWithValidJsonapi(): void
     {
         $jsonapi = '{"meta":{}}';
 
@@ -66,7 +68,7 @@ public function testIsValidResponseBodyWithValidJsonapi()
     /**
      * @test isValidResponseBody() with invalid jsonapi
      */
-    public function testIsValidResponseBodyWithInvalidJsonapi()
+    public function testIsValidResponseBodyWithInvalidJsonapi(): void
     {
         $invalid_jsonapi = '["This is valid JSON", "but invalid JSON API"]';
 
@@ -76,7 +78,7 @@ public function testIsValidResponseBodyWithInvalidJsonapi()
     /**
      * @test isValidResponseBody() with invalid json
      */
-    public function testIsValidResponseBodyWithInvalidJson()
+    public function testIsValidResponseBodyWithInvalidJson(): void
     {
         $invalid_json = 'invalid_json_string';
 
@@ -86,7 +88,7 @@ public function testIsValidResponseBodyWithInvalidJson()
     /**
      * @test parseRequestBody() with valid JSON API returns Document Object
      */
-    public function testParseRequestBodyWithValidJsonapiReturnsDocument()
+    public function testParseRequestBodyWithValidJsonapiReturnsDocument(): void
     {
         $jsonapi = '{"meta":{}}';
 
@@ -96,7 +98,7 @@ public function testParseRequestBodyWithValidJsonapiReturnsDocument()
     /**
      * @test parseRequestBody() throw Exception if input is invalid jsonapi
      */
-    public function testParseRequestBodyWithInvalidJsonapiThrowsException()
+    public function testParseRequestBodyWithInvalidJsonapiThrowsException(): void
     {
         $invalid_jsonapi = '["This is valid JSON", "but invalid JSON API"]';
 
@@ -111,7 +113,7 @@ public function testParseRequestBodyWithInvalidJsonapiThrowsException()
     /**
      * JSON API documents are defined in JavaScript Object Notation (JSON) [RFC4627].
      */
-    public function testParseRequestBodyWithInvalidJsonThrowsException()
+    public function testParseRequestBodyWithInvalidJsonThrowsException(): void
     {
         $invalid_json = 'invalid_json_string';
 
@@ -126,7 +128,7 @@ public function testParseRequestBodyWithInvalidJsonThrowsException()
     /**
      * @test isValidRequestBody() with valid JSON API returns true
      */
-    public function testIsValidRequestBodyWithValidJsonapi()
+    public function testIsValidRequestBodyWithValidJsonapi(): void
     {
         $jsonapi = '{"meta":{}}';
 
@@ -136,7 +138,7 @@ public function testIsValidRequestBodyWithValidJsonapi()
     /**
      * @test isValidRequestBody() with invalid jsonapi
      */
-    public function testIsValidRequestBodyWithInvalidJsonapi()
+    public function testIsValidRequestBodyWithInvalidJsonapi(): void
     {
         $invalid_jsonapi = '["This is valid JSON", "but invalid JSON API"]';
 
@@ -146,7 +148,7 @@ public function testIsValidRequestBodyWithInvalidJsonapi()
     /**
      * @test isValidRequestBody() with invalid json
      */
-    public function testIsValidRequestBodyWithInvalidJson()
+    public function testIsValidRequestBodyWithInvalidJson(): void
     {
         $invalid_json = 'invalid_json_string';
 
diff --git a/tests/Unit/Input/RequestStringInputTest.php b/tests/Unit/Input/RequestStringInputTest.php
index 632a639..5256afa 100644
--- a/tests/Unit/Input/RequestStringInputTest.php
+++ b/tests/Unit/Input/RequestStringInputTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -18,7 +20,7 @@ class RequestStringInputTest extends TestCase
     /**
      * @test
      */
-    public function testGetAsObjectFromStringReturnsObject()
+    public function testGetAsObjectFromStringReturnsObject(): void
     {
         $input = new RequestStringInput('{}');
 
@@ -31,7 +33,7 @@ public function testGetAsObjectFromStringReturnsObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutStringThrowsException($input)
+    public function testCreateWithoutStringThrowsException($input): void
     {
         $this->expectException(InputException::class);
         $this->expectExceptionMessage(
@@ -46,7 +48,7 @@ public function testCreateWithoutStringThrowsException($input)
      *
      * @param string $input
      */
-    public function testGetAsObjectWithInvalidStringsThrowsException(string $input)
+    public function testGetAsObjectWithInvalidStringsThrowsException(string $input): void
     {
         $input = new RequestStringInput($input);
 
diff --git a/tests/Unit/Input/ResponseStringInputTest.php b/tests/Unit/Input/ResponseStringInputTest.php
index 4087c4a..c554f1b 100644
--- a/tests/Unit/Input/ResponseStringInputTest.php
+++ b/tests/Unit/Input/ResponseStringInputTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -18,7 +20,7 @@ class ResponseStringInputTest extends TestCase
     /**
      * @test
      */
-    public function testGetAsObjectFromStringReturnsObject()
+    public function testGetAsObjectFromStringReturnsObject(): void
     {
         $input = new ResponseStringInput('{}');
 
@@ -31,7 +33,7 @@ public function testGetAsObjectFromStringReturnsObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutStringThrowsException($input)
+    public function testCreateWithoutStringThrowsException($input): void
     {
         $this->expectException(InputException::class);
         $this->expectExceptionMessage(
@@ -46,7 +48,7 @@ public function testCreateWithoutStringThrowsException($input)
      *
      * @param string $input
      */
-    public function testGetAsObjectWithInvalidStringsThrowsException(string $input)
+    public function testGetAsObjectWithInvalidStringsThrowsException(string $input): void
     {
         $input = new ResponseStringInput($input);
 
diff --git a/tests/Unit/Manager/ErrorAbortManagerTest.php b/tests/Unit/Manager/ErrorAbortManagerTest.php
index b3e8ee3..9cae683 100644
--- a/tests/Unit/Manager/ErrorAbortManagerTest.php
+++ b/tests/Unit/Manager/ErrorAbortManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -15,7 +17,7 @@ class ErrorAbortManagerTest extends TestCase
     /**
      * @test
      */
-    public function testCreateWithConstructorReturnsSelf()
+    public function testCreateWithConstructorReturnsSelf(): void
     {
         $factory = $this->createMock(Factory::class);
         $manager = new ErrorAbortManager($factory);
@@ -26,7 +28,7 @@ public function testCreateWithConstructorReturnsSelf()
     /**
      * @test
      */
-    public function testGetParamReturnsDefault()
+    public function testGetParamReturnsDefault(): void
     {
         $factory = $this->createMock(Factory::class);
         $manager = new ErrorAbortManager($factory);
diff --git a/tests/Unit/Serializer/ArraySerializerTest.php b/tests/Unit/Serializer/ArraySerializerTest.php
index 85ee62a..e4b165c 100644
--- a/tests/Unit/Serializer/ArraySerializerTest.php
+++ b/tests/Unit/Serializer/ArraySerializerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -15,7 +17,7 @@ class ArraySerializerTest extends TestCase
     /**
      * @test non-recursive serialize()
      */
-    public function testSerialize()
+    public function testSerialize(): void
     {
         $object1 = $this->createMock(Accessable::class);
         $object2 = new \stdClass();
@@ -56,7 +58,7 @@ public function testSerialize()
     /**
      * @test recursive serialize()
      */
-    public function testRecursiveSerialize()
+    public function testRecursiveSerialize(): void
     {
         $stdObject = new \stdClass();
         $stdObject->key = 'value';
diff --git a/tests/Unit/V1/AttributesTest.php b/tests/Unit/V1/AttributesTest.php
index 1d03205..865197c 100644
--- a/tests/Unit/V1/AttributesTest.php
+++ b/tests/Unit/V1/AttributesTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->object = new \stdClass();
@@ -51,15 +53,15 @@ public function testCreateWithObject()
 
         $this->assertInstanceOf(Accessable::class, $attributes);
         $this->assertTrue($attributes->has('object'));
-        $this->assertTrue(is_object($attributes->get('object')));
+        $this->assertIsObject($attributes->get('object'));
         $this->assertTrue($attributes->has('array'));
-        $this->assertTrue(is_array($attributes->get('array')));
+        $this->assertIsArray($attributes->get('array'));
         $this->assertTrue($attributes->has('string'));
-        $this->assertTrue(is_string($attributes->get('string')));
+        $this->assertIsString($attributes->get('string'));
         $this->assertTrue($attributes->has('number_int'));
-        $this->assertTrue(is_int($attributes->get('number_int')));
+        $this->assertIsInt($attributes->get('number_int'));
         $this->assertTrue($attributes->has('number_float'));
-        $this->assertTrue(is_float($attributes->get('number_float')));
+        $this->assertIsFloat($attributes->get('number_float'));
         $this->assertTrue($attributes->has('true'));
         $this->assertTrue($attributes->get('true'));
         $this->assertTrue($attributes->has('false'));
@@ -74,7 +76,7 @@ public function testCreateWithObject()
             'number_float',
             'true',
             'false',
-            'null'
+            'null',
         ], $attributes->getKeys());
     }
 
@@ -83,7 +85,7 @@ public function testCreateWithObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataProvider($input)
+    public function testCreateWithDataProvider($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -102,7 +104,7 @@ public function testCreateWithDataProvider($input)
      *
      * These properties are not allowed in attributes: `type`, `id`, `relationships`, `links`
      */
-    public function testCreateWithTypePropertyThrowsException()
+    public function testCreateWithTypePropertyThrowsException(): void
     {
         $object = new \stdClass();
         $object->type = 'posts';
@@ -124,7 +126,7 @@ public function testCreateWithTypePropertyThrowsException()
      *
      * These properties are not allowed in attributes: `type`, `id`, `relationships`, `links`
      */
-    public function testCreateWithIdPropertyThrowsException()
+    public function testCreateWithIdPropertyThrowsException(): void
     {
         $object = new \stdClass();
         $object->id = '5';
@@ -146,7 +148,7 @@ public function testCreateWithIdPropertyThrowsException()
      *
      * These properties are not allowed in attributes: `type`, `id`, `relationships`, `links`
      */
-    public function testCreateWithRelationshipsPropertyThrowsException()
+    public function testCreateWithRelationshipsPropertyThrowsException(): void
     {
         $object = new \stdClass();
         $object->relationships = new \stdClass();
@@ -168,7 +170,7 @@ public function testCreateWithRelationshipsPropertyThrowsException()
      *
      * These properties are not allowed in attributes: `type`, `id`, `relationships`, `links`
      */
-    public function testCreateWithLinksPropertyThrowsException()
+    public function testCreateWithLinksPropertyThrowsException(): void
     {
         $object = new \stdClass();
         $object->links = new \stdClass();
@@ -189,7 +191,7 @@ public function testCreateWithLinksPropertyThrowsException()
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->pages = '1126';
diff --git a/tests/Unit/V1/DocumentLinkTest.php b/tests/Unit/V1/DocumentLinkTest.php
index 7583f9a..6227032 100644
--- a/tests/Unit/V1/DocumentLinkTest.php
+++ b/tests/Unit/V1/DocumentLinkTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -41,7 +43,7 @@ public function setUp(): void
      * - related: a related resource link when the primary data represents a resource relationship.
      * - pagination links for the primary data.
      */
-    public function testOnlySelfRelatedPaginationPropertiesExists()
+    public function testOnlySelfRelatedPaginationPropertiesExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -65,7 +67,7 @@ public function testOnlySelfRelatedPaginationPropertiesExists()
             'prev',
             'next',
             'custom',
-            'meta'
+            'meta',
         ], $link->getKeys());
 
         $this->assertTrue($link->has('custom'));
@@ -93,7 +95,7 @@ public function testOnlySelfRelatedPaginationPropertiesExists()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -110,7 +112,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
+    public function testCreateWithoutObjectOrStringAttributeThrowsException($input): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -131,7 +133,7 @@ public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testSelfMustBeAStringOrObject($input)
+    public function testSelfMustBeAStringOrObject($input): void
     {
         $object = new \stdClass();
         $object->self = $input;
@@ -163,7 +165,7 @@ public function testSelfMustBeAStringOrObject($input)
      *
      * @param mixed $input
      */
-    public function testRelatedMustBeAStringOrObject($input)
+    public function testRelatedMustBeAStringOrObject($input): void
     {
         $object = new \stdClass();
         $object->related = $input;
@@ -183,7 +185,7 @@ public function testRelatedMustBeAStringOrObject($input)
      *
      * @param mixed $input
      */
-    public function testFirstCanBeAnObjectOrStringOrNull($input)
+    public function testFirstCanBeAnObjectOrStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -233,7 +235,7 @@ public function testFirstCanBeAnObjectOrStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testLastCanBeAStringOrNull($input)
+    public function testLastCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -283,7 +285,7 @@ public function testLastCanBeAStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testPrevCanBeAStringOrNull($input)
+    public function testPrevCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -333,7 +335,7 @@ public function testPrevCanBeAStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testNextCanBeAStringOrNull($input)
+    public function testNextCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -379,7 +381,7 @@ public function testNextCanBeAStringOrNull($input)
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
diff --git a/tests/Unit/V1/DocumentTest.php b/tests/Unit/V1/DocumentTest.php
index 57588ae..5a8e949 100644
--- a/tests/Unit/V1/DocumentTest.php
+++ b/tests/Unit/V1/DocumentTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object returns self
      */
-    public function testCreateWithObjectReturnsSelf()
+    public function testCreateWithObjectReturnsSelf(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -54,7 +56,7 @@ public function testCreateWithObjectReturnsSelf()
     /**
      * @test create with all possible values
      */
-    public function testCreateWithAllPossibleValues()
+    public function testCreateWithAllPossibleValues(): void
     {
         $object = new \stdClass();
         $object->data = new \stdClass();
@@ -89,7 +91,7 @@ public function testCreateWithAllPossibleValues()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataproviderThrowsException($input)
+    public function testCreateWithDataproviderThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -104,7 +106,7 @@ public function testCreateWithDataproviderThrowsException($input)
      *
      * A document MUST contain at least one of the following top-level members: data, errors, meta
      */
-    public function testCreateWithoutAnyToplevelMemberThrowsException()
+    public function testCreateWithoutAnyToplevelMemberThrowsException(): void
     {
         $object = new \stdClass();
 
@@ -124,7 +126,7 @@ public function testCreateWithoutAnyToplevelMemberThrowsException()
      * - a single resource object, a single resource identifier object, or null, for requests that target single resources
      * - an array of resource objects, an array of resource identifier objects, or an empty array ([]), for requests that target resource collections
      */
-    public function testCreateDataWithResourceIdentifier()
+    public function testCreateDataWithResourceIdentifier(): void
     {
         $data = new \stdClass();
         $data->type = 'posts';
@@ -144,7 +146,7 @@ public function testCreateDataWithResourceIdentifier()
     /**
      * @test create with data resource identifier and meta
      */
-    public function testCreateDataWithResourceIdentifierAndMeta()
+    public function testCreateDataWithResourceIdentifierAndMeta(): void
     {
         $data = new \stdClass();
         $data->type = 'posts';
@@ -165,7 +167,7 @@ public function testCreateDataWithResourceIdentifierAndMeta()
     /**
      * @test create with data resource item
      */
-    public function testCreateDataWithResourceItem()
+    public function testCreateDataWithResourceItem(): void
     {
         $data = new \stdClass();
         $data->type = 'posts';
@@ -187,7 +189,7 @@ public function testCreateDataWithResourceItem()
     /**
      * @test create with data object array
      */
-    public function testCreateDataWithResourceCollectionIdentifiers()
+    public function testCreateDataWithResourceCollectionIdentifiers(): void
     {
         $data_obj = new \stdClass();
         $data_obj->type = 'types';
@@ -207,7 +209,7 @@ public function testCreateDataWithResourceCollectionIdentifiers()
     /**
      * @test create with data object array advanced
      */
-    public function testCreateDataWithResourceCollectionResources()
+    public function testCreateDataWithResourceCollectionResources(): void
     {
         $data_obj = new \stdClass();
         $data_obj->type = 'types';
@@ -233,7 +235,7 @@ public function testCreateDataWithResourceCollectionResources()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataproviderInValue($input)
+    public function testCreateWithDataproviderInValue($input): void
     {
 
         // Test with empty array in data
@@ -282,7 +284,7 @@ public function testCreateWithDataproviderInValue($input)
     /**
      * @test create with an errors array
      */
-    public function testCreateWithErrorsArray()
+    public function testCreateWithErrorsArray(): void
     {
         $object = new \stdClass();
         $object->errors = [
@@ -302,7 +304,7 @@ public function testCreateWithErrorsArray()
     /**
      * @test The members `data` and `errors` MUST NOT coexist in the same document.
      */
-    public function testCreateWithDataAndErrorsThrowsException()
+    public function testCreateWithDataAndErrorsThrowsException(): void
     {
         $object = new \stdClass();
         $object->data = new \stdClass();
@@ -319,7 +321,7 @@ public function testCreateWithDataAndErrorsThrowsException()
     /**
      * @test create with meta object
      */
-    public function testCreateWithMetaObject()
+    public function testCreateWithMetaObject(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -335,7 +337,7 @@ public function testCreateWithMetaObject()
     /**
      * @test create with Jsonapi object
      */
-    public function testCreateWithJsonapiObject()
+    public function testCreateWithJsonapiObject(): void
     {
         $object = new \stdClass();
 
@@ -353,7 +355,7 @@ public function testCreateWithJsonapiObject()
     /**
      * @test create with link object
      */
-    public function testCreateWithLinkObject()
+    public function testCreateWithLinkObject(): void
     {
         $object = new \stdClass();
 
@@ -371,7 +373,7 @@ public function testCreateWithLinkObject()
     /**
      * @test create with included objects
      */
-    public function testCreateWithIncludedObjects()
+    public function testCreateWithIncludedObjects(): void
     {
         $data = new \stdClass();
         $data->type = 'posts';
@@ -397,7 +399,7 @@ public function testCreateWithIncludedObjects()
     /**
      * @test If a document does not contain a top-level `data` key, the `included` member MUST NOT be present either.
      */
-    public function testCreateIncludedWithoutDataThrowsException()
+    public function testCreateIncludedWithoutDataThrowsException(): void
     {
         $object = new \stdClass();
         $object->included = new \stdClass();
@@ -414,7 +416,7 @@ public function testCreateIncludedWithoutDataThrowsException()
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
diff --git a/tests/Unit/V1/ErrorCollectionTest.php b/tests/Unit/V1/ErrorCollectionTest.php
index eb6a376..31b2731 100644
--- a/tests/Unit/V1/ErrorCollectionTest.php
+++ b/tests/Unit/V1/ErrorCollectionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create
      */
-    public function testCreate()
+    public function testCreate(): void
     {
         $errors = [
             new \stdClass(),
@@ -65,7 +67,7 @@ public function testCreate()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutArrayThrowsException($input)
+    public function testCreateWithoutArrayThrowsException($input): void
     {
         // Input must be an array with at least one object
         if (gettype($input) === 'array') {
@@ -86,7 +88,7 @@ public function testCreateWithoutArrayThrowsException($input)
     /**
      * @test get('resources') on an empty collection throws an exception
      */
-    public function testGetErrorWithEmptyCollectionThrowsException()
+    public function testGetErrorWithEmptyCollectionThrowsException(): void
     {
         $errors = [
             new \stdClass(),
diff --git a/tests/Unit/V1/ErrorLinkTest.php b/tests/Unit/V1/ErrorLinkTest.php
index e916163..7b66639 100644
--- a/tests/Unit/V1/ErrorLinkTest.php
+++ b/tests/Unit/V1/ErrorLinkTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -35,7 +37,7 @@ public function setUp(): void
      * - links: a links object containing the following members:
      *   - about: a link that leads to further details about this particular occurrence of the problem.
      */
-    public function testAllPropertiesExists()
+    public function testAllPropertiesExists(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -63,7 +65,7 @@ public function testAllPropertiesExists()
      * - links: a links object containing the following members:
      *   - about: a link that leads to further details about this particular occurrence of the problem.
      */
-    public function testAboutMustBeSet()
+    public function testAboutMustBeSet(): void
     {
         $object = new \stdClass();
         $object->foobar = new \stdClass();
@@ -83,7 +85,7 @@ public function testAboutMustBeSet()
      * - links: a links object containing the following members:
      *   - about: a link that leads to further details about this particular occurrence of the problem.
      */
-    public function testAboutCanBeAnObject()
+    public function testAboutCanBeAnObject(): void
     {
         $object = new \stdClass();
         $object->about = new \stdClass();
@@ -104,7 +106,7 @@ public function testAboutCanBeAnObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataprovider($input)
+    public function testCreateWithDataprovider($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -121,7 +123,7 @@ public function testCreateWithDataprovider($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
+    public function testCreateWithoutObjectOrStringAttributeThrowsException($input): void
     {
         $object = new \stdClass();
         $object->about = 'http://example.org/about';
@@ -142,7 +144,7 @@ public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testAboutWithDataproviderThrowsException($input)
+    public function testAboutWithDataproviderThrowsException($input): void
     {
         $object = new \stdClass();
         $object->about = $input;
@@ -158,7 +160,7 @@ public function testAboutWithDataproviderThrowsException($input)
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->about = 'http://example.org/about';
diff --git a/tests/Unit/V1/ErrorSourceTest.php b/tests/Unit/V1/ErrorSourceTest.php
index 77dbce5..553a99e 100644
--- a/tests/Unit/V1/ErrorSourceTest.php
+++ b/tests/Unit/V1/ErrorSourceTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -35,7 +37,7 @@ public function setUp(): void
      * - pointer: a JSON Pointer [RFC6901] to the associated entity in the request document [e.g. "/data" for a primary data object, or "/data/attributes/title" for a specific attribute].
      * - parameter: a string indicating which query parameter caused the error.
      */
-    public function testOnlyPointerParameterPropertiesExists()
+    public function testOnlyPointerParameterPropertiesExists(): void
     {
         $object = new \stdClass();
         $object->pointer = '/pointer';
@@ -72,7 +74,7 @@ public function testOnlyPointerParameterPropertiesExists()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -89,7 +91,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testPointerMustBeAString($input)
+    public function testPointerMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->pointer = $input;
@@ -109,7 +111,7 @@ public function testPointerMustBeAString($input)
      *
      * @param mixed $input
      */
-    public function testParameterMustBeAString($input)
+    public function testParameterMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->parameter = $input;
diff --git a/tests/Unit/V1/ErrorTest.php b/tests/Unit/V1/ErrorTest.php
index b41443e..a777c85 100644
--- a/tests/Unit/V1/ErrorTest.php
+++ b/tests/Unit/V1/ErrorTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object returns self
      */
-    public function testCreateWithObjectReturnsSelf()
+    public function testCreateWithObjectReturnsSelf(): void
     {
         $object = new \stdClass();
         $object->id = 'id';
@@ -83,7 +85,7 @@ public function testCreateWithObjectReturnsSelf()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -98,7 +100,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateIdWithoutStringThrowsException($input)
+    public function testCreateIdWithoutStringThrowsException($input): void
     {
         $object = new \stdClass();
         $object->id = $input;
@@ -116,7 +118,7 @@ public function testCreateIdWithoutStringThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateStatusWithoutStringThrowsException($input)
+    public function testCreateStatusWithoutStringThrowsException($input): void
     {
         $object = new \stdClass();
         $object->status = $input;
@@ -134,7 +136,7 @@ public function testCreateStatusWithoutStringThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateCodeWithoutStringThrowsException($input)
+    public function testCreateCodeWithoutStringThrowsException($input): void
     {
         $object = new \stdClass();
         $object->code = $input;
@@ -152,7 +154,7 @@ public function testCreateCodeWithoutStringThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateTitleWithoutStringThrowsException($input)
+    public function testCreateTitleWithoutStringThrowsException($input): void
     {
         $object = new \stdClass();
         $object->title = $input;
@@ -170,7 +172,7 @@ public function testCreateTitleWithoutStringThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateDetailWithoutStringThrowsException($input)
+    public function testCreateDetailWithoutStringThrowsException($input): void
     {
         $object = new \stdClass();
         $object->detail = $input;
diff --git a/tests/Unit/V1/FactoryTest.php b/tests/Unit/V1/FactoryTest.php
index 5d1b6b0..ea2b356 100644
--- a/tests/Unit/V1/FactoryTest.php
+++ b/tests/Unit/V1/FactoryTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -20,7 +22,7 @@ class FactoryTest extends TestCase
     /**
      * @test
      */
-    public function testInjectACustomClass()
+    public function testInjectACustomClass(): void
     {
         $factory = new Factory([
             'Default' => ResourceNull::class,
@@ -39,7 +41,7 @@ public function testInjectACustomClass()
     /**
      * @test parse throw Exception if input is invalid jsonapi
      */
-    public function testMakeAnUndefindedClassThrowsException()
+    public function testMakeAnUndefindedClassThrowsException(): void
     {
         $factory = new Factory();
 
@@ -51,7 +53,7 @@ public function testMakeAnUndefindedClassThrowsException()
         $class = $factory->make('NotExistent');
     }
 
-    public function testMakeWithClassNotImplementingAccessableThrowsException()
+    public function testMakeWithClassNotImplementingAccessableThrowsException(): void
     {
         $factory = new Factory([
             'Default' => stdClass::class,
diff --git a/tests/Unit/V1/JsonapiTest.php b/tests/Unit/V1/JsonapiTest.php
index 853dc02..ccbeb0a 100644
--- a/tests/Unit/V1/JsonapiTest.php
+++ b/tests/Unit/V1/JsonapiTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->version = '1.0';
@@ -74,7 +76,7 @@ public function testCreateWithObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataprovider($input)
+    public function testCreateWithDataprovider($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -91,7 +93,7 @@ public function testCreateWithDataprovider($input)
      *
      * @param mixed $input
      */
-    public function testVersionCannotBeAnObjectOrArray($input)
+    public function testVersionCannotBeAnObjectOrArray($input): void
     {
         $object = new \stdClass();
         $object->version = $input;
@@ -114,6 +116,6 @@ public function testVersionCannotBeAnObjectOrArray($input)
 
         // other input must be transformed to string
         $this->assertTrue($jsonapi->has('version'));
-        $this->assertTrue(is_string($jsonapi->get('version')));
+        $this->assertIsString($jsonapi->get('version'));
     }
 }
diff --git a/tests/Unit/V1/LinkTest.php b/tests/Unit/V1/LinkTest.php
index f07ccfc..e62d237 100644
--- a/tests/Unit/V1/LinkTest.php
+++ b/tests/Unit/V1/LinkTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -68,7 +70,7 @@ public function testCreateWithObject()
      *
      * @param mixed $input
      */
-    public function testHrefHasToBeAString($input)
+    public function testHrefHasToBeAString($input): void
     {
         $object = new \stdClass();
         $object->href = $input;
@@ -77,7 +79,7 @@ public function testHrefHasToBeAString($input)
             $link = new Link($object, $this->manager, $this->parent);
 
             $this->assertTrue($link->has('href'));
-            $this->assertTrue(is_string($link->get('href')));
+            $this->assertIsString($link->get('href'));
 
             return;
         }
@@ -96,7 +98,7 @@ public function testHrefHasToBeAString($input)
      * - an object ("link object") which can contain the following members:
      *   - href: a string containing the link's URL.
      */
-    public function testHrefAttributeMustBeSet()
+    public function testHrefAttributeMustBeSet(): void
     {
         $object = new \stdClass();
         $object->related = 'http://example.org/related';
@@ -112,7 +114,7 @@ public function testHrefAttributeMustBeSet()
     /**
      * @test meta attribute will be parsed as Meta object inside Link
      */
-    public function testMetaIsParsedAsObject()
+    public function testMetaIsParsedAsObject(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -131,7 +133,7 @@ public function testMetaIsParsedAsObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithDataprovider($input)
+    public function testCreateWithDataprovider($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
diff --git a/tests/Unit/V1/MetaTest.php b/tests/Unit/V1/MetaTest.php
index e7f4f11..218ed5f 100644
--- a/tests/Unit/V1/MetaTest.php
+++ b/tests/Unit/V1/MetaTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->object = new \stdClass();
@@ -51,15 +53,15 @@ public function testCreateWithObject()
 
         $this->assertInstanceOf(Accessable::class, $meta);
         $this->assertTrue($meta->has('object'));
-        $this->assertTrue(is_object($meta->get('object')));
+        $this->assertIsObject($meta->get('object'));
         $this->assertTrue($meta->has('array'));
-        $this->assertTrue(is_array($meta->get('array')));
+        $this->assertIsArray($meta->get('array'));
         $this->assertTrue($meta->has('string'));
-        $this->assertTrue(is_string($meta->get('string')));
+        $this->assertIsString($meta->get('string'));
         $this->assertTrue($meta->has('number_int'));
-        $this->assertTrue(is_int($meta->get('number_int')));
+        $this->assertIsInt($meta->get('number_int'));
         $this->assertTrue($meta->has('number_float'));
-        $this->assertTrue(is_float($meta->get('number_float')));
+        $this->assertIsFloat($meta->get('number_float'));
         $this->assertTrue($meta->has('true'));
         $this->assertTrue($meta->get('true'));
         $this->assertTrue($meta->has('false'));
@@ -80,7 +82,7 @@ public function testCreateWithObject()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage('Meta has to be an object, "' . gettype($input) . '" given.');
@@ -91,7 +93,7 @@ public function testCreateWithoutObjectThrowsException($input)
     /**
      * @test get() with not existing key throws an exception
      */
-    public function testGetWithNotExistingKeyThrowsException()
+    public function testGetWithNotExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
 
diff --git a/tests/Unit/V1/RelationshipCollectionTest.php b/tests/Unit/V1/RelationshipCollectionTest.php
index ca2981f..fa6ef95 100644
--- a/tests/Unit/V1/RelationshipCollectionTest.php
+++ b/tests/Unit/V1/RelationshipCollectionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->author = new \stdClass();
@@ -66,7 +68,7 @@ public function testCreateWithObject()
     /**
      * @test create with empty object
      */
-    public function testCreateWithEmptyObject()
+    public function testCreateWithEmptyObject(): void
     {
         $item = $this->createMock(Accessable::class);
 
@@ -86,7 +88,7 @@ public function testCreateWithEmptyObject()
      *
      * Fields for a resource object MUST share a common namespace with each other and with `type` and `id`.
      */
-    public function testCreateWithTypePropertyThrowsException()
+    public function testCreateWithTypePropertyThrowsException(): void
     {
         $item = $this->createMock(Accessable::class);
 
@@ -111,7 +113,7 @@ public function testCreateWithTypePropertyThrowsException()
      *
      * Fields for a resource object MUST share a common namespace with each other and with `type` and `id`.
      */
-    public function testCreateWithIdPropertyThrowsException()
+    public function testCreateWithIdPropertyThrowsException(): void
     {
         $item = $this->createMock(Accessable::class);
 
@@ -136,7 +138,7 @@ public function testCreateWithIdPropertyThrowsException()
      *
      * In other words, a resource can not have an attribute and relationship with the same name,
      */
-    public function testCreateWithAuthorInRelationshipsAndAttributesThrowsException()
+    public function testCreateWithAuthorInRelationshipsAndAttributesThrowsException(): void
     {
         $item = $this->createMock(Accessable::class);
 
@@ -161,7 +163,7 @@ public function testCreateWithAuthorInRelationshipsAndAttributesThrowsException(
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
diff --git a/tests/Unit/V1/RelationshipLinkTest.php b/tests/Unit/V1/RelationshipLinkTest.php
index 18b9090..475e080 100644
--- a/tests/Unit/V1/RelationshipLinkTest.php
+++ b/tests/Unit/V1/RelationshipLinkTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -59,7 +61,7 @@ public function setUp(): void
      * A relationship object that represents a to-many relationship MAY also contain pagination
      * links under the links member, as described below.
      */
-    public function testOnlySelfRelatedPaginationPropertiesExists()
+    public function testOnlySelfRelatedPaginationPropertiesExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -98,7 +100,7 @@ public function testOnlySelfRelatedPaginationPropertiesExists()
     /**
      * @test pagination links are parsed, if data in parent relationship object exists
      */
-    public function testPaginationParsedIfRelationshipDataExists()
+    public function testPaginationParsedIfRelationshipDataExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -118,7 +120,7 @@ public function testPaginationParsedIfRelationshipDataExists()
     /**
      * @test pagination links are not parsed, if data in parent relationship object doesnt exist
      */
-    public function testPaginationNotParsedIfRelationshipDataNotExists()
+    public function testPaginationNotParsedIfRelationshipDataNotExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -155,7 +157,7 @@ public function testPaginationNotParsedIfRelationshipDataNotExists()
     /**
      * @test pagination links are not parsed, if data in parent relationship object is not IdentifierCollection
      */
-    public function testPaginationNotParsedIfRelationshipIdentifierCollectionNotExists()
+    public function testPaginationNotParsedIfRelationshipIdentifierCollectionNotExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -204,7 +206,7 @@ public function testPaginationNotParsedIfRelationshipIdentifierCollectionNotExis
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -221,7 +223,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
+    public function testCreateWithoutObjectOrStringAttributeThrowsException($input): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -238,7 +240,7 @@ public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
     /**
      * @test object contains at least one of the following: self, related
      */
-    public function testCreateWithoutSelfAndRelatedPropertiesThrowsException()
+    public function testCreateWithoutSelfAndRelatedPropertiesThrowsException(): void
     {
         $object = new \stdClass();
         $object->first = 'http://example.org/first';
@@ -259,7 +261,7 @@ public function testCreateWithoutSelfAndRelatedPropertiesThrowsException()
      *
      * @param mixed $input
      */
-    public function testSelfMustBeAStringOrObject($input)
+    public function testSelfMustBeAStringOrObject($input): void
     {
         $object = new \stdClass();
         $object->self = $input;
@@ -280,7 +282,7 @@ public function testSelfMustBeAStringOrObject($input)
      *
      * @param mixed $input
      */
-    public function testRelatedMustBeAStringOrObject($input)
+    public function testRelatedMustBeAStringOrObject($input): void
     {
         $object = new \stdClass();
         $object->related = $input;
@@ -300,7 +302,7 @@ public function testRelatedMustBeAStringOrObject($input)
      *
      * @param mixed $input
      */
-    public function testFirstCanBeAStringOrNull($input)
+    public function testFirstCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -341,7 +343,7 @@ public function testFirstCanBeAStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testLastCanBeAStringOrNull($input)
+    public function testLastCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -382,7 +384,7 @@ public function testLastCanBeAStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testPrevCanBeAStringOrNull($input)
+    public function testPrevCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -423,7 +425,7 @@ public function testPrevCanBeAStringOrNull($input)
      *
      * @param mixed $input
      */
-    public function testNextCanBeAStringOrNull($input)
+    public function testNextCanBeAStringOrNull($input): void
     {
         $object = new \stdClass();
         $object->self = 'https://example.org/self';
@@ -460,7 +462,7 @@ public function testNextCanBeAStringOrNull($input)
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
diff --git a/tests/Unit/V1/RelationshipTest.php b/tests/Unit/V1/RelationshipTest.php
index 948e2bc..7574890 100644
--- a/tests/Unit/V1/RelationshipTest.php
+++ b/tests/Unit/V1/RelationshipTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object returns self
      */
-    public function testCreateWithObjectReturnsSelf()
+    public function testCreateWithObjectReturnsSelf(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
@@ -63,7 +65,7 @@ public function testCreateWithObjectReturnsSelf()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -78,7 +80,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * A "relationship object" MUST contain at least one of the following: links, data, meta
      */
-    public function testCreateWithoutLinksDataMetaPropertiesThrowsException()
+    public function testCreateWithoutLinksDataMetaPropertiesThrowsException(): void
     {
         $object = new \stdClass();
         $object->foo = 'bar';
@@ -94,7 +96,7 @@ public function testCreateWithoutLinksDataMetaPropertiesThrowsException()
     /**
      * @test create with link object
      */
-    public function testCreateWithLinksObject()
+    public function testCreateWithLinksObject(): void
     {
         $object = new \stdClass();
         $object->links = new \stdClass();
@@ -117,7 +119,7 @@ public function testCreateWithLinksObject()
      *
      * data: resource linkage, see http://jsonapi.org/format/#document-resource-object-linkage
      */
-    public function testCreateWithDataObject()
+    public function testCreateWithDataObject(): void
     {
         $data = new \stdClass();
         $data->type = 'posts';
@@ -139,7 +141,7 @@ public function testCreateWithDataObject()
     /**
      * @test create with data null
      */
-    public function testCreateWithDataNull()
+    public function testCreateWithDataNull(): void
     {
         $object = new \stdClass();
         $object->data = null;
@@ -151,13 +153,13 @@ public function testCreateWithDataNull()
         $this->assertSame($relationship->getKeys(), ['data']);
         $this->assertTrue($relationship->has('data'));
 
-        $this->assertTrue(is_null($relationship->get('data')));
+        $this->assertNull($relationship->get('data'));
     }
 
     /**
      * @test create with data object array
      */
-    public function testCreateWithDataObjectArray()
+    public function testCreateWithDataObjectArray(): void
     {
         $data_obj = new \stdClass();
         $data_obj->type = 'types';
@@ -181,7 +183,7 @@ public function testCreateWithDataObjectArray()
     /**
      * @test create with data empty array
      */
-    public function testCreateWithDataEmptyArray()
+    public function testCreateWithDataEmptyArray(): void
     {
         $object = new \stdClass();
         $object->data = [];
@@ -201,7 +203,7 @@ public function testCreateWithDataEmptyArray()
     /**
      * @test create with meta object
      */
-    public function testCreateWithMetaObject()
+    public function testCreateWithMetaObject(): void
     {
         $object = new \stdClass();
         $object->meta = new \stdClass();
diff --git a/tests/Unit/V1/ResourceCollectionTest.php b/tests/Unit/V1/ResourceCollectionTest.php
index 616f3d5..67d00e1 100644
--- a/tests/Unit/V1/ResourceCollectionTest.php
+++ b/tests/Unit/V1/ResourceCollectionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with empty array
      */
-    public function testCreateWithEmptyArray()
+    public function testCreateWithEmptyArray(): void
     {
         $collection = new ResourceCollection([], $this->manager, $this->parent);
 
@@ -50,7 +52,7 @@ public function testCreateWithEmptyArray()
     /**
      * @test create with identifier object
      */
-    public function testCreateWithIdentifier()
+    public function testCreateWithIdentifier(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -71,7 +73,7 @@ public function testCreateWithIdentifier()
     /**
      * @test create with identifier object and meta
      */
-    public function testCreateWithIdentifierAndMeta()
+    public function testCreateWithIdentifierAndMeta(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -107,7 +109,7 @@ public function testCreateWithIdentifierAndMeta()
     /**
      * @test create with item object
      */
-    public function testCreateWithItem()
+    public function testCreateWithItem(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -133,7 +135,7 @@ public function testCreateWithItem()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutArrayThrowsException($input)
+    public function testCreateWithoutArrayThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -148,7 +150,7 @@ public function testCreateWithoutArrayThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectInArrayThrowsException($input)
+    public function testCreateWithoutObjectInArrayThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -161,7 +163,7 @@ public function testCreateWithoutObjectInArrayThrowsException($input)
     /**
      * @test get('resources') on an empty collection throws an exception
      */
-    public function testGetResourcesWithEmptyCollectionThrowsException()
+    public function testGetResourcesWithEmptyCollectionThrowsException(): void
     {
         $collection = new ResourceCollection([], $this->manager, $this->createMock(Accessable::class));
 
diff --git a/tests/Unit/V1/ResourceIdentifierCollectionTest.php b/tests/Unit/V1/ResourceIdentifierCollectionTest.php
index bffe9e1..3ab6f02 100644
--- a/tests/Unit/V1/ResourceIdentifierCollectionTest.php
+++ b/tests/Unit/V1/ResourceIdentifierCollectionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with empty array
      */
-    public function testCreateWithEmptyArray()
+    public function testCreateWithEmptyArray(): void
     {
         $collection = new ResourceIdentifierCollection([], $this->manager, $this->parent);
 
@@ -45,7 +47,7 @@ public function testCreateWithEmptyArray()
     /**
      * @test create with identifier object
      */
-    public function testCreateWithIdentifier()
+    public function testCreateWithIdentifier(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -71,7 +73,7 @@ public function testCreateWithIdentifier()
     /**
      * @test create with item object
      */
-    public function testCreateWithItem()
+    public function testCreateWithItem(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -98,7 +100,7 @@ public function testCreateWithItem()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutArrayThrowsException($input)
+    public function testCreateWithoutArrayThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -111,7 +113,7 @@ public function testCreateWithoutArrayThrowsException($input)
     /**
      * @test get('resources') on an empty identifier collection throws an exception
      */
-    public function testGetResourcesWithEmptyCollectionThrowsException()
+    public function testGetResourcesWithEmptyCollectionThrowsException(): void
     {
         $collection = new ResourceIdentifierCollection([], $this->manager, $this->parent);
 
diff --git a/tests/Unit/V1/ResourceIdentifierTest.php b/tests/Unit/V1/ResourceIdentifierTest.php
index 58f6673..65e4ff5 100644
--- a/tests/Unit/V1/ResourceIdentifierTest.php
+++ b/tests/Unit/V1/ResourceIdentifierTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -51,7 +53,7 @@ public function testCreateWithObject()
     /**
      * @test create with object and meta
      */
-    public function testCreateWithObjectAndMeta()
+    public function testCreateWithObjectAndMeta(): void
     {
         $object = new \stdClass();
         $object->type = 'types';
@@ -76,7 +78,7 @@ public function testCreateWithObjectAndMeta()
      *
      * @param mixed $input
      */
-    public function testTypeMustBeAString($input)
+    public function testTypeMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->type = $input;
@@ -89,7 +91,7 @@ public function testTypeMustBeAString($input)
 
         $identifier = new ResourceIdentifier($object, $this->manager, $this->parent);
 
-        $this->assertTrue(is_string($identifier->get('type')));
+        $this->assertIsString($identifier->get('type'));
     }
 
     /**
@@ -99,7 +101,7 @@ public function testTypeMustBeAString($input)
      *
      * @param mixed $input
      */
-    public function testIdMustBeAString($input)
+    public function testIdMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->type = 'posts';
@@ -121,7 +123,7 @@ public function testIdMustBeAString($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithDataproviderThrowsException($input)
+    public function testCreateWithDataproviderThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -134,7 +136,7 @@ public function testCreateWithDataproviderThrowsException($input)
     /**
      * @test A "resource identifier object" MUST contain type and id members.
      */
-    public function testCreateWithObjectWithoutTypeThrowsException()
+    public function testCreateWithObjectWithoutTypeThrowsException(): void
     {
         $object = new \stdClass();
         $object->id = '123';
@@ -150,7 +152,7 @@ public function testCreateWithObjectWithoutTypeThrowsException()
     /**
      * @test A "resource identifier object" MUST contain type and id members.
      */
-    public function testCreateWithObjectWithoutIdThrowsException()
+    public function testCreateWithObjectWithoutIdThrowsException(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -166,7 +168,7 @@ public function testCreateWithObjectWithoutIdThrowsException()
     /**
      * @test get() on an undefined value throws Exception
      */
-    public function testGetWithUndefinedValueThrowsException()
+    public function testGetWithUndefinedValueThrowsException(): void
     {
         $object = new \stdClass();
         $object->type = 'posts';
diff --git a/tests/Unit/V1/ResourceItemLinkTest.php b/tests/Unit/V1/ResourceItemLinkTest.php
index b86d5ad..1d98f6f 100644
--- a/tests/Unit/V1/ResourceItemLinkTest.php
+++ b/tests/Unit/V1/ResourceItemLinkTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test parsing of all properties
      */
-    public function testParsingPropertiesExists()
+    public function testParsingPropertiesExists(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
@@ -59,7 +61,7 @@ public function testParsingPropertiesExists()
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectThrowsException($input)
+    public function testCreateWithoutObjectThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -76,7 +78,7 @@ public function testCreateWithoutObjectThrowsException($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
+    public function testCreateWithoutObjectOrStringAttributeThrowsException($input): void
     {
         $object = new \stdClass();
         $object->input = $input;
@@ -92,7 +94,7 @@ public function testCreateWithoutObjectOrStringAttributeThrowsException($input)
     /**
      * @test
      */
-    public function testGetOnANonExistingKeyThrowsException()
+    public function testGetOnANonExistingKeyThrowsException(): void
     {
         $object = new \stdClass();
         $object->self = 'http://example.org/self';
diff --git a/tests/Unit/V1/ResourceItemTest.php b/tests/Unit/V1/ResourceItemTest.php
index 63db0db..8641760 100644
--- a/tests/Unit/V1/ResourceItemTest.php
+++ b/tests/Unit/V1/ResourceItemTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -31,7 +33,7 @@ public function setUp(): void
     /**
      * @test create with object
      */
-    public function testCreateWithObject()
+    public function testCreateWithObject(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -66,7 +68,7 @@ public function testCreateWithObject()
     /**
      * @test create with full object
      */
-    public function testCreateWithFullObject()
+    public function testCreateWithFullObject(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
@@ -100,7 +102,7 @@ public function testCreateWithFullObject()
      *
      * @param mixed $input
      */
-    public function testTypeMustBeAString($input)
+    public function testTypeMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->type = $input;
@@ -121,7 +123,7 @@ public function testTypeMustBeAString($input)
      *
      * @param mixed $input
      */
-    public function testIdMustBeAString($input)
+    public function testIdMustBeAString($input): void
     {
         $object = new \stdClass();
         $object->type = 'posts';
@@ -143,7 +145,7 @@ public function testIdMustBeAString($input)
      *
      * @param mixed $input
      */
-    public function testCreateWithDataproviderThrowsException($input)
+    public function testCreateWithDataproviderThrowsException($input): void
     {
         $this->expectException(ValidationException::class);
         $this->expectExceptionMessage(
@@ -156,7 +158,7 @@ public function testCreateWithDataproviderThrowsException($input)
     /**
      * @test A "resource object" MUST contain type and id members.
      */
-    public function testCreateWithObjectWithoutTypeThrowsException()
+    public function testCreateWithObjectWithoutTypeThrowsException(): void
     {
         $object = new \stdClass();
         $object->id = 123;
@@ -172,7 +174,7 @@ public function testCreateWithObjectWithoutTypeThrowsException()
     /**
      * @test A "resource object" MUST contain type and id members.
      */
-    public function testCreateWithObjectWithoutIdThrowsException()
+    public function testCreateWithObjectWithoutIdThrowsException(): void
     {
         $object = new \stdClass();
         $object->type = 'type';
diff --git a/tests/Unit/V1/ResourceNullTest.php b/tests/Unit/V1/ResourceNullTest.php
index e470600..cbaf377 100644
--- a/tests/Unit/V1/ResourceNullTest.php
+++ b/tests/Unit/V1/ResourceNullTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 // SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
@@ -32,7 +34,7 @@ public function setUp(): void
      *
      * @param mixed $input
      */
-    public function testCreateWithDataProvider($input)
+    public function testCreateWithDataProvider($input): void
     {
         $resource = new ResourceNull(
             $input,
@@ -49,7 +51,7 @@ public function testCreateWithDataProvider($input)
     /**
      * @test get throws Exception
      */
-    public function testGetThrowsException()
+    public function testGetThrowsException(): void
     {
         $resource = new ResourceNull(
             null,