diff --git a/.phpstan.neon b/.phpstan.neon
index 8136274..5567202 100644
--- a/.phpstan.neon
+++ b/.phpstan.neon
@@ -1,32 +1,78 @@
 # SPDX-FileCopyrightText: 2015-2023 Artur Weigandt https://wlabs.de/kontakt
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+includes:
+	- vendor/phpstan/phpstan-phpunit/extension.neon
+
 parameters:
-	level: 8
+	level: 9
 
 	paths:
 		- src/
+		- tests/
 
 	scanDirectories:
 		- vendor
 
+	treatPhpDocTypesAsCertain: false
+
 	ignoreErrors:
 		-
-			message: "#^Class Art4\\\\JsonApiClient\\\\Helper\\\\AccessKey extends generic class SplStack but does not specify its types\\: TValue$#"
+			message: "#^Constructor of class Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull has an unused parameter \\$data\\.$#"
 			count: 1
-			path: src/Helper/AccessKey.php
+			path: src/V1/ResourceNull.php
+			# parameter is required by Art4\JsonApiClient\Element
 
 		-
-			message: "#^Property Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull\\:\\:\\$data is never read, only written\\.$#"
+			message: "#^Constructor of class Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull has an unused parameter \\$manager\\.$#"
 			count: 1
 			path: src/V1/ResourceNull.php
+			# parameter is required by Art4\JsonApiClient\Element
 
 		-
-			message: "#^Property Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull\\:\\:\\$manager is never read, only written\\.$#"
+			message: "#^Constructor of class Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull has an unused parameter \\$parent\\.$#"
 			count: 1
 			path: src/V1/ResourceNull.php
+			# parameter is required by Art4\JsonApiClient\Element
 
 		-
-			message: "#^Property Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull\\:\\:\\$parent is never read, only written\\.$#"
+			message: "#^Constructor of an anonymous class has an unused parameter \\$data\\.$#"
 			count: 1
-			path: src/V1/ResourceNull.php
+			path: tests/BC/ElementTest.php
+			# parameter is required by Art4\JsonApiClient\Element
+
+		-
+			message: "#^Constructor of an anonymous class has an unused parameter \\$manager\\.$#"
+			count: 1
+			path: tests/BC/ElementTest.php
+			# parameter is required by Art4\JsonApiClient\Element
+
+		-
+			message: "#^Constructor of an anonymous class has an unused parameter \\$parent\\.$#"
+			count: 1
+			path: tests/BC/ElementTest.php
+			# parameter is required by Art4\JsonApiClient\Element
+
+		-
+			message: "#^Parameter \\#1 \\$string of class Art4\\\\JsonApiClient\\\\Input\\\\RequestStringInput constructor expects string, mixed given\\.$#"
+			count: 1
+			path: tests/Unit/Input/RequestStringInputTest.php
+			# We are providing an invalid parameter to test the exception message
+
+		-
+			message: "#^Parameter \\#1 \\$string of class Art4\\\\JsonApiClient\\\\Input\\\\ResponseStringInput constructor expects string, mixed given\\.$#"
+			count: 1
+			path: tests/Unit/Input/ResponseStringInputTest.php
+			# We are providing an invalid parameter to test the exception message
+
+		-
+			message: "#^Parameter \\#1 \\$key of method Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull\\:\\:has\\(\\) expects Art4\\\\JsonApiClient\\\\Helper\\\\AccessKey\\|int\\|string, array given\\.$#"
+			count: 1
+			path: tests/Unit/V1/ResourceNullTest.php
+			# We are providing an invalid parameter to test the deprecation message
+
+		-
+			message: "#^Parameter \\#1 \\$key of method Art4\\\\JsonApiClient\\\\V1\\\\ResourceNull\\:\\:has\\(\\) expects Art4\\\\JsonApiClient\\\\Helper\\\\AccessKey\\|int\\|string, stdClass given\\.$#"
+			count: 1
+			path: tests/Unit/V1/ResourceNullTest.php
+			# We are providing an invalid parameter to test the deprecation message
diff --git a/composer.json b/composer.json
index 82738e1..48f9de4 100644
--- a/composer.json
+++ b/composer.json
@@ -18,6 +18,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.35",
         "phpstan/phpstan": "^1.10",
+        "phpstan/phpstan-phpunit": "^1.3",
         "phpunit/phpunit": "^9 || ^10"
     },
     "autoload": {
diff --git a/src/Accessable.php b/src/Accessable.php
index ec46ab1..cf63145 100644
--- a/src/Accessable.php
+++ b/src/Accessable.php
@@ -8,6 +8,8 @@
 
 namespace Art4\JsonApiClient;
 
+use Art4\JsonApiClient\Helper\AccessKey;
+
 /**
  * Accessable Interface
  */
@@ -39,7 +41,7 @@ public function has($key);
      *
      * @deprecated `\Art4\JsonApiClient\Accessable::getKeys()` will add `array` as a native return type declaration in v2.0. Do the same in your implementation now to avoid errors.
      *
-     * @return array<string> Keys of all setted values
+     * @return array<string|int> Keys of all setted values
      */
     public function getKeys();
     // public function getKeys(): array;
diff --git a/src/Helper/AccessKey.php b/src/Helper/AccessKey.php
index a57558f..bed0120 100644
--- a/src/Helper/AccessKey.php
+++ b/src/Helper/AccessKey.php
@@ -13,6 +13,8 @@
 /**
  * AccessKey
  *
+ * @extends SplStack<string>
+ *
  * @internal
  */
 final class AccessKey extends SplStack
@@ -20,7 +22,7 @@ final class AccessKey extends SplStack
     /**
      * Transforms the Key to a string
      *
-     * @param mixed $key
+     * @param int|string $key
      *
      * @return AccessKey<string>
      */
diff --git a/src/Helper/AccessableTrait.php b/src/Helper/AccessableTrait.php
index 81eb8bf..6cee963 100644
--- a/src/Helper/AccessableTrait.php
+++ b/src/Helper/AccessableTrait.php
@@ -41,7 +41,7 @@ final protected function set(string $key, $value): void
     /**
      * Returns the keys of all setted values
      *
-     * @return array<string> Keys of all setted values
+     * @return array<int|string> Keys of all setted values
      */
     final public function getKeys(): array
     {
@@ -51,10 +51,19 @@ final public function getKeys(): array
     /**
      * Check if a value exists
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      */
     final public function has($key): bool
     {
+        if (! is_int($key) && ! is_string($key) && (! is_object($key) || ! $key instanceof AccessKey)) {
+            trigger_error(sprintf(
+                '%s::has(): Providing Argument #1 ($key) as %s is deprecated since 1.2.0, please provide as int|string|%s instead.',
+                get_class($this),
+                gettype($key),
+                AccessKey::class
+            ), \E_USER_DEPRECATED);
+        }
+
         $key = $this->parseKey($key);
 
         $string = $key->shift();
@@ -82,7 +91,7 @@ final public function has($key): bool
     /**
      * Get a value by a key
      *
-     * @param mixed $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return mixed
      */
@@ -126,7 +135,7 @@ private function getValue(string $key)
     /**
      * Parse a dot.notated.key to an object
      *
-     * @param string|AccessKey<string> $key The key
+     * @param int|string|AccessKey<string> $key The key
      *
      * @return AccessKey<string> The parsed key
      */
diff --git a/src/Serializer/ArraySerializer.php b/src/Serializer/ArraySerializer.php
index 42d1491..422037f 100644
--- a/src/Serializer/ArraySerializer.php
+++ b/src/Serializer/ArraySerializer.php
@@ -35,7 +35,7 @@ public function __construct(array $params = [])
     /**
      * Convert data in an array
      *
-     * @return array<string, mixed>|null
+     * @return array<int|string, mixed>|null
      */
     public function serialize(Accessable $data): ?array
     {
diff --git a/src/V1/Attributes.php b/src/V1/Attributes.php
index 918dcf3..2f4a660 100644
--- a/src/V1/Attributes.php
+++ b/src/V1/Attributes.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Attributes Object
@@ -50,7 +51,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Document.php b/src/V1/Document.php
index ff3194d..4e971cb 100644
--- a/src/V1/Document.php
+++ b/src/V1/Document.php
@@ -9,9 +9,10 @@
 namespace Art4\JsonApiClient\V1;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Document Top Level Object
@@ -73,7 +74,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/DocumentLink.php b/src/V1/DocumentLink.php
index 30ab3c3..4d0dc0b 100644
--- a/src/V1/DocumentLink.php
+++ b/src/V1/DocumentLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Document Link Object
@@ -103,7 +104,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Error.php b/src/V1/Error.php
index 69c2f7b..346cd55 100644
--- a/src/V1/Error.php
+++ b/src/V1/Error.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Object
@@ -105,7 +106,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorCollection.php b/src/V1/ErrorCollection.php
index 42def2f..b61e0f2 100644
--- a/src/V1/ErrorCollection.php
+++ b/src/V1/ErrorCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Collection Object
@@ -44,7 +45,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this document
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorLink.php b/src/V1/ErrorLink.php
index 112d09b..6c08772 100644
--- a/src/V1/ErrorLink.php
+++ b/src/V1/ErrorLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Link Object
@@ -63,7 +64,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ErrorSource.php b/src/V1/ErrorSource.php
index 3c70b99..e8136e2 100644
--- a/src/V1/ErrorSource.php
+++ b/src/V1/ErrorSource.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Error Source Object
@@ -52,7 +53,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this document
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Jsonapi.php b/src/V1/Jsonapi.php
index db56bef..6a5acbb 100644
--- a/src/V1/Jsonapi.php
+++ b/src/V1/Jsonapi.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * JSON API Object
@@ -48,7 +49,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Link.php b/src/V1/Link.php
index 1c07bae..5e585b1 100644
--- a/src/V1/Link.php
+++ b/src/V1/Link.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Link Object
@@ -44,7 +45,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
@@ -60,8 +61,7 @@ public function get($key)
     /**
      * Set a link
      *
-     * @param string        $name The Name
-     * @param string|object $link The Link
+     * @param mixed $link The Link
      */
     private function setAsLink(string $name, $link): void
     {
diff --git a/src/V1/Meta.php b/src/V1/Meta.php
index 5836bab..db1dcc1 100644
--- a/src/V1/Meta.php
+++ b/src/V1/Meta.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Meta Object
@@ -46,7 +47,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/Relationship.php b/src/V1/Relationship.php
index 5a842cf..ec6b0a4 100644
--- a/src/V1/Relationship.php
+++ b/src/V1/Relationship.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -59,7 +60,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/RelationshipCollection.php b/src/V1/RelationshipCollection.php
index 49680f0..fd9c9dc 100644
--- a/src/V1/RelationshipCollection.php
+++ b/src/V1/RelationshipCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Relationship Collection Object
@@ -54,7 +55,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/RelationshipLink.php b/src/V1/RelationshipLink.php
index 84b1c59..3cb303f 100644
--- a/src/V1/RelationshipLink.php
+++ b/src/V1/RelationshipLink.php
@@ -11,6 +11,7 @@
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
 use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Relationship Link Object
@@ -105,7 +106,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceCollection.php b/src/V1/ResourceCollection.php
index 4577e2b..2b515e0 100644
--- a/src/V1/ResourceCollection.php
+++ b/src/V1/ResourceCollection.php
@@ -9,9 +9,10 @@
 namespace Art4\JsonApiClient\V1;
 
 use Art4\JsonApiClient\Accessable;
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Object
@@ -47,7 +48,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceIdentifier.php b/src/V1/ResourceIdentifier.php
index 26e6b2c..df39116 100644
--- a/src/V1/ResourceIdentifier.php
+++ b/src/V1/ResourceIdentifier.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -59,7 +60,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceIdentifierCollection.php b/src/V1/ResourceIdentifierCollection.php
index a887070..e820d37 100644
--- a/src/V1/ResourceIdentifierCollection.php
+++ b/src/V1/ResourceIdentifierCollection.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Object
@@ -42,7 +43,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceItem.php b/src/V1/ResourceItem.php
index caae85a..35c43b0 100644
--- a/src/V1/ResourceItem.php
+++ b/src/V1/ResourceItem.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Resource Identifier Object
@@ -77,7 +78,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceItemLink.php b/src/V1/ResourceItemLink.php
index 69ce850..5315593 100644
--- a/src/V1/ResourceItemLink.php
+++ b/src/V1/ResourceItemLink.php
@@ -8,9 +8,10 @@
 
 namespace Art4\JsonApiClient\V1;
 
-use Art4\JsonApiClient\Helper\AbstractElement;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AbstractElement;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * ItemLink Object
@@ -40,7 +41,7 @@ protected function parse($object): void
     /**
      * Get a value by the key of this object
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return mixed The value
      */
diff --git a/src/V1/ResourceNull.php b/src/V1/ResourceNull.php
index c7df660..3d48c84 100644
--- a/src/V1/ResourceNull.php
+++ b/src/V1/ResourceNull.php
@@ -12,17 +12,13 @@
 use Art4\JsonApiClient\Element;
 use Art4\JsonApiClient\Manager;
 use Art4\JsonApiClient\Exception\AccessException;
+use Art4\JsonApiClient\Helper\AccessKey;
 
 /**
  * Null Resource
  */
 final class ResourceNull implements Accessable, Element
 {
-    /** @var mixed */
-    private $data;
-    private Manager $manager;
-    private Accessable $parent;
-
     /**
      * Constructor
      *
@@ -30,22 +26,26 @@ final class ResourceNull implements Accessable, Element
      * @param \Art4\JsonApiClient\Manager    $manager The manager
      * @param \Art4\JsonApiClient\Accessable $parent  The parent
      */
-    public function __construct($data, Manager $manager, Accessable $parent)
-    {
-        $this->data = $data;
-        $this->manager = $manager;
-        $this->parent = $parent;
-    }
+    public function __construct($data, Manager $manager, Accessable $parent) {}
 
     /**
      * Check if a value exists in this resource
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      *
      * @return bool false
      */
     public function has($key)
     {
+        if (! is_int($key) && ! is_string($key) && (! is_object($key) || ! $key instanceof AccessKey)) {
+            trigger_error(sprintf(
+                '%s::has(): Providing Argument #1 ($key) as %s is deprecated since 1.2.0, please provide as int|string|%s instead.',
+                get_class($this),
+                gettype($key),
+                AccessKey::class
+            ), \E_USER_DEPRECATED);
+        }
+
         return false;
     }
 
@@ -62,7 +62,7 @@ public function getKeys()
     /**
      * Get a value by the key of this identifier
      *
-     * @param string $key The key of the value
+     * @param int|string|AccessKey<string> $key The key of the value
      */
     public function get($key): void
     {
diff --git a/tests/Fixtures/Factory.php b/tests/Fixtures/Factory.php
deleted file mode 100644
index b6d916c..0000000
--- a/tests/Fixtures/Factory.php
+++ /dev/null
@@ -1,35 +0,0 @@
-<?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\Tests\Fixtures;
-
-use Art4\JsonApiClient\Utils\FactoryInterface;
-
-final class Factory implements FactoryInterface
-{
-    public $testcase;
-
-    /**
-     * Create a new instance of a class
-     *
-     * @param string $name
-     * @param array  $args
-     *
-     * @return object
-     */
-    public function make($name, array $args = [])
-    {
-        return $this->testcase
-            ->getMockBuilder('Art4\JsonApiClient\\' . $name . 'Interface') // Mock only the interfaces
-            ->disableOriginalConstructor()
-            ->disableOriginalClone()
-            ->disableArgumentCloning()
-            ->disallowMockingUnknownTypes()
-            ->getMock();
-    }
-}
diff --git a/tests/Fixtures/HelperTrait.php b/tests/Fixtures/HelperTrait.php
index 94cc0d1..aa0b604 100644
--- a/tests/Fixtures/HelperTrait.php
+++ b/tests/Fixtures/HelperTrait.php
@@ -11,23 +11,26 @@
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Factory;
 use Art4\JsonApiClient\Manager;
-use Art4\JsonApiClient\Tests\Fixtures\Factory as FixtureFactory;
+use PHPUnit\Framework\MockObject\MockObject;
 
 /**
  * Helper Trait
  */
 trait HelperTrait
 {
+    /** @var Manager&MockObject */
     protected Manager $manager;
 
+    /** @var Factory&MockObject */
     protected Factory $factory;
 
+    /** @var Accessable&MockObject */
     protected Accessable $parent;
 
     /**
      * Json Values Provider
      *
-     * @see http://json.org/
+     * @return array<array<mixed>>
      */
     public static function jsonValuesProvider(): array
     {
@@ -47,7 +50,7 @@ public static function jsonValuesProvider(): array
     /**
      * Json Values Provider but without the object
      *
-     * @see http://json.org/
+     * @return array<array<mixed>>
      */
     public static function jsonValuesProviderWithoutObject(): array
     {
@@ -61,7 +64,7 @@ public static function jsonValuesProviderWithoutObject(): array
     /**
      * Json Values Provider but without the array
      *
-     * @see http://json.org/
+     * @return array<array<mixed>>
      */
     public static function jsonValuesProviderWithoutArray(): array
     {
@@ -75,7 +78,7 @@ public static function jsonValuesProviderWithoutArray(): array
     /**
      * Json Values Provider but without the string
      *
-     * @see http://json.org/
+     * @return array<array<mixed>>
      */
     public static function jsonValuesProviderWithoutString(): array
     {
@@ -89,7 +92,7 @@ public static function jsonValuesProviderWithoutString(): array
     /**
      * Json Values Provider but without the object and string
      *
-     * @see http://json.org/
+     * @return array<array<mixed>>
      */
     public static function jsonValuesProviderWithoutObjectAndString(): array
     {
@@ -104,7 +107,7 @@ public static function jsonValuesProviderWithoutObjectAndString(): array
     /**
      * Json Values as string Provider
      *
-     * @see http://json.org/
+     * @return array<array<string>>
      */
     public static function jsonValuesAsStringProvider(): array
     {
@@ -124,7 +127,7 @@ public static function jsonValuesAsStringProvider(): array
     /**
      * Json Values as string Provider but without the object
      *
-     * @see http://json.org/
+     * @return array<array<string>>
      */
     public static function jsonValuesAsStringProviderWithoutObject(): array
     {
@@ -135,30 +138,6 @@ public static function jsonValuesAsStringProviderWithoutObject(): array
         return array_values($data);
     }
 
-    /**
-     * Builds a Manager Mock
-     */
-    public function buildManagerMock()
-    {
-        // Mock factory
-        $factory = new FixtureFactory();
-        $factory->testcase = $this;
-
-        // Mock Manager
-        $manager = $this->createMock('Art4\JsonApiClient\Utils\FactoryManagerInterface');
-
-        $manager->expects($this->any())
-            ->method('getFactory')
-            ->will($this->returnValue($factory));
-
-        $manager->expects($this->any())
-            ->method('getConfig')
-            ->with('optional_item_id')
-            ->willReturn(false);
-
-        return $manager;
-    }
-
     /**
      * Builds a Manager Mock and set it into the TestCase
      */
@@ -185,8 +164,8 @@ public function setUpManagerMock(): void
      *
      * @param mixed $filename
      */
-    protected function getJsonString($filename)
+    protected function getJsonString($filename): string
     {
-        return file_get_contents(__DIR__ . '/../files/' . $filename);
+        return strval(file_get_contents(__DIR__ . '/../files/' . $filename));
     }
 }
diff --git a/tests/Fixtures/V1Factory.php b/tests/Fixtures/V1Factory.php
index 282ea05..835c047 100644
--- a/tests/Fixtures/V1Factory.php
+++ b/tests/Fixtures/V1Factory.php
@@ -8,31 +8,26 @@
 
 namespace Art4\JsonApiClient\Tests\Fixtures;
 
-use Art4\JsonApiClient\Element;
 use Art4\JsonApiClient\Factory as FactoryInterface;
+use PHPUnit\Framework\TestCase;
 
 final class V1Factory implements FactoryInterface
 {
-    public $testcase;
+    public TestCase $testcase;
 
     /**
      * Create a factory
-     *
-     * @param object $testcase
-     * @param array  $args
-     *
-     * @return object
      */
-    public function __construct($testcase)
+    public function __construct(TestCase $testcase)
     {
-        return $this->testcase = $testcase;
+        $this->testcase = $testcase;
     }
 
     /**
      * Create a new instance of a class
      *
      * @param string $name
-     * @param array  $args
+     * @param array<mixed> $args
      *
      * @return object
      */
diff --git a/tests/Functional/ParsingTest.php b/tests/Functional/ParsingTest.php
index 40565e1..7e4957c 100644
--- a/tests/Functional/ParsingTest.php
+++ b/tests/Functional/ParsingTest.php
@@ -22,6 +22,8 @@ class ParsingTest extends TestCase
 
     /**
      * Provide Parser
+     *
+     * @return array<array<\Closure>>
      */
     public static function createParserProvider(): array
     {
@@ -37,12 +39,9 @@ public static function createParserProvider(): array
     }
 
     /**
-     * @test
      * @dataProvider createParserProvider
-     *
-     * @param mixed $parser
      */
-    public function testParseSimpleResourceWithDifferentParser($parser): void
+    public function testParseSimpleResourceWithDifferentParser(callable $parser): void
     {
         $string = $this->getJsonString('01_simple_resource.json');
         $document = $parser($string);
@@ -176,6 +175,7 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($document->has('included'));
         $this->assertTrue($document->has('data'));
 
+        /** @var Accessable */
         $resources = $document->get('data');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceCollection', $resources);
@@ -184,6 +184,8 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertSame($resources->getKeys(), [0]);
 
         $this->assertTrue($resources->has(0));
+
+        /** @var Accessable */
         $resource = $resources->get(0);
 
         $this->assertFalse($resource->has('meta'));
@@ -193,24 +195,28 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($resource->has('relationships'));
         $this->assertTrue($resource->has('links'));
 
+        /** @var Accessable */
         $attributes = $resource->get('attributes');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Attributes', $attributes);
         $this->assertTrue($attributes->has('title'));
         $this->assertSame($attributes->get('title'), 'JSON API paints my bikeshed!');
 
+        /** @var Accessable */
         $collection = $resource->get('relationships');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\RelationshipCollection', $collection);
         $this->assertTrue($collection->has('author'));
         $this->assertTrue($collection->has('comments'));
 
+        /** @var Accessable */
         $author = $collection->get('author');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Relationship', $author);
         $this->assertTrue($author->has('links'));
         $this->assertTrue($author->has('data'));
 
+        /** @var Accessable */
         $links = $author->get('links');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\RelationshipLink', $links);
@@ -219,18 +225,21 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($links->has('related'));
         $this->assertSame($links->get('related'), 'http://example.com/articles/1/author');
 
+        /** @var Accessable */
         $data = $author->get('data');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceIdentifier', $data);
         $this->assertSame($data->get('type'), 'people');
         $this->assertSame($data->get('id'), '9');
 
+        /** @var Accessable */
         $comments = $collection->get('comments');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Relationship', $comments);
         $this->assertTrue($comments->has('links'));
         $this->assertTrue($comments->has('data'));
 
+        /** @var Accessable */
         $links = $comments->get('links');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\RelationshipLink', $links);
@@ -239,35 +248,42 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($links->has('related'));
         $this->assertSame($links->get('related'), 'http://example.com/articles/1/comments');
 
+        /** @var Accessable */
         $data_array = $comments->get('data');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceIdentifierCollection', $data_array);
         $this->assertCount(2, $data_array->getKeys());
 
+        /** @var Accessable */
         $identifier = $data_array->get(0);
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceIdentifier', $identifier);
         $this->assertSame($identifier->get('type'), 'comments');
         $this->assertSame($identifier->get('id'), '5');
 
+        /** @var Accessable */
         $identifier = $data_array->get(1);
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceIdentifier', $identifier);
         $this->assertSame($identifier->get('type'), 'comments');
         $this->assertSame($identifier->get('id'), '12');
 
+        /** @var Accessable */
         $links = $resource->get('links');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceItemLink', $links);
         $this->assertTrue($links->has('self'));
         $this->assertSame($links->get('self'), 'http://example.com/articles/1');
 
+        /** @var Accessable */
         $includes = $document->get('included');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceCollection', $includes);
         $this->assertSame($includes->getKeys(), [0, 1, 2]);
 
         $this->assertTrue($includes->has(0));
+
+        /** @var Accessable */
         $include = $includes->get(0);
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceItem', $include);
@@ -276,6 +292,7 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($include->has('attributes'));
         $this->assertTrue($include->has('links'));
 
+        /** @var Accessable */
         $attributes = $include->get('attributes');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Attributes', $attributes);
@@ -286,6 +303,7 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertTrue($attributes->has('twitter'));
         $this->assertSame($attributes->get('twitter'), 'dgeb');
 
+        /** @var Accessable */
         $links = $include->get('links');
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceItemLink', $links);
@@ -293,6 +311,8 @@ public function testParseCompleteResourceObjectWithMultipleRelationships(): void
         $this->assertSame($links->get('self'), 'http://example.com/people/9');
 
         $this->assertTrue($includes->has(1));
+
+        /** @var Accessable */
         $include = $includes->get(1);
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\ResourceItem', $include);
diff --git a/tests/Functional/SerializerTest.php b/tests/Functional/SerializerTest.php
index f96c043..c835969 100644
--- a/tests/Functional/SerializerTest.php
+++ b/tests/Functional/SerializerTest.php
@@ -14,6 +14,7 @@
 use Art4\JsonApiClient\Serializer\ArraySerializer;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\Factory;
+use Exception;
 use PHPUnit\Framework\TestCase;
 
 class SerializerTest extends TestCase
@@ -22,6 +23,8 @@ class SerializerTest extends TestCase
 
     /**
      * Provide JSON API data
+     *
+     * @return array<array<string|bool>>
      */
     public static function jsonapiDataProvider(): array
     {
@@ -33,7 +36,13 @@ public static function jsonapiDataProvider(): array
             '15_create_resource_without_id.json',
         ];
 
-        foreach (glob($path . '*.json') as $file) {
+        $filenames = glob($path . '*.json');
+
+        if ($filenames === false) {
+            throw new Exception('Error while getting all json files.');
+        }
+
+        foreach ($filenames as $file) {
             $filename = str_replace($path, '', $file);
 
             // Ignore files with errors
@@ -45,9 +54,7 @@ public static function jsonapiDataProvider(): array
 
             $files[] = [
                 $filename,
-                [
-                    'is_request' => in_array($filename, $requestFiles),
-                ],
+                in_array($filename, $requestFiles),
             ];
         }
 
@@ -55,18 +62,15 @@ public static function jsonapiDataProvider(): array
     }
 
     /**
-     * @test
      * @dataProvider jsonapiDataProvider
-     *
-     * @param mixed $filename
      */
-    public function parseJsonapiDataWithErrorAbortManager($filename, array $meta): void
+    public function testParseJsonapiDataWithErrorAbortManager(string $filename, bool $isRequest): void
     {
         $manager = new ErrorAbortManager(new Factory());
 
         $string = $this->getJsonString($filename);
 
-        if ($meta['is_request']) {
+        if ($isRequest) {
             $input = new RequestStringInput($string);
         } else {
             $input = new ResponseStringInput($string);
diff --git a/tests/Unit/Helper/AccessableTraitTest.php b/tests/Unit/Helper/AccessableTraitTest.php
new file mode 100644
index 0000000..cda7d40
--- /dev/null
+++ b/tests/Unit/Helper/AccessableTraitTest.php
@@ -0,0 +1,60 @@
+<?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\Tests\Unit\Helper;
+
+use Art4\JsonApiClient\Helper\AccessableTrait;
+use Art4\JsonApiClient\Accessable;
+use PHPUnit\Framework\TestCase;
+
+class AccessableTraitTest extends TestCase
+{
+    public function testHasWithObjectAsKeyTriggersException(): void
+    {
+        /** @var Accessable */
+        $resource = $this->getMockForTrait(AccessableTrait::class);
+
+        // PHPUnit 10 compatible way to test trigger_error().
+        set_error_handler(
+            function ($errno, $errstr): bool {
+                $this->assertStringEndsWith(
+                    '::has(): Providing Argument #1 ($key) as object is deprecated since 1.2.0, please provide as int|string|Art4\JsonApiClient\Helper\AccessKey instead.',
+                    $errstr
+                );
+
+                restore_error_handler();
+                return true;
+            },
+            E_USER_DEPRECATED
+        );
+
+        $resource->has(new \stdClass());
+    }
+
+    public function testHasWithArrayAsKeyTriggersException(): void
+    {
+        /** @var Accessable */
+        $resource = $this->getMockForTrait(AccessableTrait::class);
+
+        // PHPUnit 10 compatible way to test trigger_error().
+        set_error_handler(
+            function ($errno, $errstr): bool {
+                $this->assertStringEndsWith(
+                    '::has(): Providing Argument #1 ($key) as array is deprecated since 1.2.0, please provide as int|string|Art4\JsonApiClient\Helper\AccessKey instead.',
+                    $errstr
+                );
+
+                restore_error_handler();
+                return true;
+            },
+            E_USER_DEPRECATED
+        );
+
+        $resource->has([]);
+    }
+}
diff --git a/tests/Unit/Input/RequestStringInputTest.php b/tests/Unit/Input/RequestStringInputTest.php
index 5256afa..f9d49e7 100644
--- a/tests/Unit/Input/RequestStringInputTest.php
+++ b/tests/Unit/Input/RequestStringInputTest.php
@@ -17,9 +17,6 @@ class RequestStringInputTest extends TestCase
 {
     use HelperTrait;
 
-    /**
-     * @test
-     */
     public function testGetAsObjectFromStringReturnsObject(): void
     {
         $input = new RequestStringInput('{}');
@@ -29,7 +26,6 @@ public function testGetAsObjectFromStringReturnsObject(): void
 
     /**
      * @dataProvider jsonValuesProviderWithoutString
-     * @test
      *
      * @param mixed $input
      */
@@ -44,9 +40,6 @@ public function testCreateWithoutStringThrowsException($input): void
 
     /**
      * @dataProvider jsonValuesAsStringProviderWithoutObject
-     * @test
-     *
-     * @param string $input
      */
     public function testGetAsObjectWithInvalidStringsThrowsException(string $input): void
     {
diff --git a/tests/Unit/Input/ResponseStringInputTest.php b/tests/Unit/Input/ResponseStringInputTest.php
index c554f1b..8b6f24c 100644
--- a/tests/Unit/Input/ResponseStringInputTest.php
+++ b/tests/Unit/Input/ResponseStringInputTest.php
@@ -17,9 +17,6 @@ class ResponseStringInputTest extends TestCase
 {
     use HelperTrait;
 
-    /**
-     * @test
-     */
     public function testGetAsObjectFromStringReturnsObject(): void
     {
         $input = new ResponseStringInput('{}');
@@ -29,7 +26,6 @@ public function testGetAsObjectFromStringReturnsObject(): void
 
     /**
      * @dataProvider jsonValuesProviderWithoutString
-     * @test
      *
      * @param mixed $input
      */
@@ -44,9 +40,6 @@ public function testCreateWithoutStringThrowsException($input): void
 
     /**
      * @dataProvider jsonValuesAsStringProviderWithoutObject
-     * @test
-     *
-     * @param string $input
      */
     public function testGetAsObjectWithInvalidStringsThrowsException(string $input): void
     {
diff --git a/tests/Unit/V1/ErrorCollectionTest.php b/tests/Unit/V1/ErrorCollectionTest.php
index 31b2731..eadff7d 100644
--- a/tests/Unit/V1/ErrorCollectionTest.php
+++ b/tests/Unit/V1/ErrorCollectionTest.php
@@ -11,6 +11,7 @@
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AccessKey;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\ErrorCollection;
 use PHPUnit\Framework\TestCase;
@@ -47,10 +48,10 @@ public function testCreate(): void
 
         $this->assertSame($collection->getKeys(), [0, 1]);
 
-        $this->assertFalse($collection->has(new \stdClass()));
-        $this->assertFalse($collection->has([]));
         $this->assertFalse($collection->has('string'));
 
+        $this->assertTrue($collection->has(AccessKey::create(0)));
+
         $this->assertTrue($collection->has(0));
         $error = $collection->get(0);
 
diff --git a/tests/Unit/V1/ResourceCollectionTest.php b/tests/Unit/V1/ResourceCollectionTest.php
index 67d00e1..aed1f05 100644
--- a/tests/Unit/V1/ResourceCollectionTest.php
+++ b/tests/Unit/V1/ResourceCollectionTest.php
@@ -11,6 +11,7 @@
 use Art4\JsonApiClient\Accessable;
 use Art4\JsonApiClient\Exception\AccessException;
 use Art4\JsonApiClient\Exception\ValidationException;
+use Art4\JsonApiClient\Helper\AccessKey;
 use Art4\JsonApiClient\Tests\Fixtures\HelperTrait;
 use Art4\JsonApiClient\V1\ResourceCollection;
 use PHPUnit\Framework\TestCase;
@@ -41,11 +42,10 @@ public function testCreateWithEmptyArray(): void
         $this->assertInstanceOf(Accessable::class, $collection);
 
         $this->assertSame($collection->getKeys(), []);
-        $this->assertFalse($collection->has(0));
 
         // Test get() with various key types
-        $this->assertFalse($collection->has(new \stdClass()));
-        $this->assertFalse($collection->has([]));
+        $this->assertFalse($collection->has(0));
+        $this->assertFalse($collection->has(AccessKey::create(0)));
         $this->assertFalse($collection->has('string'));
     }
 
diff --git a/tests/Unit/V1/ResourceNullTest.php b/tests/Unit/V1/ResourceNullTest.php
index cbaf377..b59c69e 100644
--- a/tests/Unit/V1/ResourceNullTest.php
+++ b/tests/Unit/V1/ResourceNullTest.php
@@ -64,4 +64,54 @@ public function testGetThrowsException(): void
 
         $resource->get('something');
     }
+
+    public function testHasWithObjectAsKeyTriggersException(): void
+    {
+        $resource = new ResourceNull(
+            null,
+            $this->manager,
+            $this->parent
+        );
+
+        // PHPUnit 10 compatible way to test trigger_error().
+        set_error_handler(
+            function ($errno, $errstr): bool {
+                $this->assertSame(
+                    'Art4\JsonApiClient\V1\ResourceNull::has(): Providing Argument #1 ($key) as object is deprecated since 1.2.0, please provide as int|string|Art4\JsonApiClient\Helper\AccessKey instead.',
+                    $errstr
+                );
+
+                restore_error_handler();
+                return true;
+            },
+            E_USER_DEPRECATED
+        );
+
+        $resource->has(new \stdClass());
+    }
+
+    public function testHasWithArrayAsKeyTriggersException(): void
+    {
+        $resource = new ResourceNull(
+            null,
+            $this->manager,
+            $this->parent
+        );
+
+        // PHPUnit 10 compatible way to test trigger_error().
+        set_error_handler(
+            function ($errno, $errstr): bool {
+                $this->assertSame(
+                    'Art4\JsonApiClient\V1\ResourceNull::has(): Providing Argument #1 ($key) as array is deprecated since 1.2.0, please provide as int|string|Art4\JsonApiClient\Helper\AccessKey instead.',
+                    $errstr
+                );
+
+                restore_error_handler();
+                return true;
+            },
+            E_USER_DEPRECATED
+        );
+
+        $resource->has([]);
+    }
 }