diff --git a/.distignore b/.distignore
index f3aedc0b..19556d45 100644
--- a/.distignore
+++ b/.distignore
@@ -2,7 +2,9 @@
 /.git
 /.github
 /.wordpress-org
+/bin
 /node_modules
+/tests
 /vendor
 
 # Files
@@ -10,10 +12,15 @@
 /.editorconfig
 /.gitattributes
 /.gitignore
+/.phpunit.result.cache
+/.sonarcloud.properties
 /.stylelintrc.json
 /composer.json
 /composer.lock
 /package.json
 /package-lock.json
 /phpcs.xml
+/phpunit.xml
+/phpunit.coverage.xml
+/phpunit.report.xml
 /README.md
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 50788e22..30b9bc6e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -1,6 +1,45 @@
-name: Coding Standards
+name: Tests
 on: [push, pull_request]
 jobs:
+  unit:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - php: '5.6'
+            wordpress: '4.7'
+          - php: '7.4'
+            wordpress: '5.9'
+          - php: '8.0'
+            wordpress: '6.0'
+          - php: '8.1'
+            wordpress: 'latest'
+          - php: '8.1'
+            wordpress: 'nightly'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{matrix.php}}
+          tools: composer
+      - name: Install
+        run: composer install --no-interaction
+      - name: Build
+        run: composer build
+      - name: Setup DB
+        uses: shogo82148/actions-setup-mysql@v1
+        with:
+          mysql-version: 'mysql-5.7'
+          root-password: "root"
+      - name: Setup WP
+        run: bash bin/install-wp-tests.sh wordpress_test root root 127.0.0.1 "${{ matrix.wordpress }}"
+      - name: PHP unit tests
+        run: composer test
+
   quality:
     runs-on: ubuntu-latest
     steps:
diff --git a/.gitignore b/.gitignore
index f5833602..d2694b1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,8 @@
 .idea/
+.phpunit.result.cache
 css/*.min.css
 vendor/
 node_modules/
 composer.lock
 package-lock.json
+phpunit.*.xml
diff --git a/.sonarcloud.properties b/.sonarcloud.properties
new file mode 100644
index 00000000..67986297
--- /dev/null
+++ b/.sonarcloud.properties
@@ -0,0 +1,9 @@
+# Path to sources.
+sonar.sources=inc,js,css,cachify.php
+sonar.exclusions=**/*.min.css,**/*.min.js
+#sonar.inclusions=
+
+# Path to tests.
+sonar.tests=tests
+#sonar.test.exclusions=
+#sonar.test.inclusions=
diff --git a/bin/install-wp-tests.sh b/bin/install-wp-tests.sh
new file mode 100644
index 00000000..b9447efd
--- /dev/null
+++ b/bin/install-wp-tests.sh
@@ -0,0 +1,163 @@
+#!/usr/bin/env bash
+
+if [ $# -lt 3 ]; then
+	echo "usage: $0 <db-name> <db-user> <db-pass> [db-host] [wp-version] [skip-database-creation]"
+	exit 1
+fi
+
+DB_NAME=$1
+DB_USER=$2
+DB_PASS=$3
+DB_HOST=${4-localhost}
+WP_VERSION=${5-latest}
+SKIP_DB_CREATE=${6-false}
+
+TMPDIR=${TMPDIR-/tmp}
+TMPDIR=$(echo $TMPDIR | sed -e "s/\/$//")
+WP_TESTS_DIR=${WP_TESTS_DIR-$TMPDIR/wordpress-tests-lib}
+WP_CORE_DIR=${WP_CORE_DIR-$TMPDIR/wordpress/}
+
+download() {
+    if [ `which curl` ]; then
+        curl -s "$1" > "$2";
+    elif [ `which wget` ]; then
+        wget -nv -O "$2" "$1"
+    fi
+}
+
+if [[ $WP_VERSION =~ ^[0-9]+\.[0-9]+$ ]]; then
+	WP_TESTS_TAG="branches/$WP_VERSION"
+elif [[ $WP_VERSION =~ [0-9]+\.[0-9]+\.[0-9]+ ]]; then
+	if [[ $WP_VERSION =~ [0-9]+\.[0-9]+\.[0] ]]; then
+		# version x.x.0 means the first release of the major version, so strip off the .0 and download version x.x
+		WP_TESTS_TAG="tags/${WP_VERSION%??}"
+	else
+		WP_TESTS_TAG="tags/$WP_VERSION"
+	fi
+elif [[ $WP_VERSION == 'nightly' || $WP_VERSION == 'trunk' ]]; then
+	WP_TESTS_TAG="trunk"
+else
+	# http serves a single offer, whereas https serves multiple. we only want one
+	download http://api.wordpress.org/core/version-check/1.7/ /tmp/wp-latest.json
+	grep '[0-9]+\.[0-9]+(\.[0-9]+)?' /tmp/wp-latest.json
+	LATEST_VERSION=$(grep -o '"version":"[^"]*' /tmp/wp-latest.json | sed 's/"version":"//')
+	if [[ -z "$LATEST_VERSION" ]]; then
+		echo "Latest WordPress version could not be found"
+		exit 1
+	fi
+	WP_TESTS_TAG="tags/$LATEST_VERSION"
+fi
+
+set -ex
+
+install_wp() {
+
+	if [ -d $WP_CORE_DIR ]; then
+		return;
+	fi
+
+	mkdir -p $WP_CORE_DIR
+
+	if [[ $WP_VERSION == 'nightly' || $WP_VERSION == 'trunk' ]]; then
+		mkdir -p $TMPDIR/wordpress-nightly
+		download https://wordpress.org/nightly-builds/wordpress-latest.zip  $TMPDIR/wordpress-nightly/wordpress-nightly.zip
+		unzip -q $TMPDIR/wordpress-nightly/wordpress-nightly.zip -d $TMPDIR/wordpress-nightly/
+		mv $TMPDIR/wordpress-nightly/wordpress/* $WP_CORE_DIR
+	else
+		if [ $WP_VERSION == 'latest' ]; then
+			local ARCHIVE_NAME='latest'
+		elif [[ $WP_VERSION =~ [0-9]+\.[0-9]+ ]]; then
+			# https serves multiple offers, whereas http serves single.
+			download https://api.wordpress.org/core/version-check/1.7/ $TMPDIR/wp-latest.json
+			if [[ $WP_VERSION =~ [0-9]+\.[0-9]+\.[0] ]]; then
+				# version x.x.0 means the first release of the major version, so strip off the .0 and download version x.x
+				LATEST_VERSION=${WP_VERSION%??}
+			else
+				# otherwise, scan the releases and get the most up to date minor version of the major release
+				local VERSION_ESCAPED=`echo $WP_VERSION | sed 's/\./\\\\./g'`
+				LATEST_VERSION=$(grep -o '"version":"'$VERSION_ESCAPED'[^"]*' $TMPDIR/wp-latest.json | sed 's/"version":"//' | head -1)
+			fi
+			if [[ -z "$LATEST_VERSION" ]]; then
+				local ARCHIVE_NAME="wordpress-$WP_VERSION"
+			else
+				local ARCHIVE_NAME="wordpress-$LATEST_VERSION"
+			fi
+		else
+			local ARCHIVE_NAME="wordpress-$WP_VERSION"
+		fi
+		download https://wordpress.org/${ARCHIVE_NAME}.tar.gz  $TMPDIR/wordpress.tar.gz
+		tar --strip-components=1 -zxmf $TMPDIR/wordpress.tar.gz -C $WP_CORE_DIR
+	fi
+
+	download https://raw.github.com/markoheijnen/wp-mysqli/master/db.php $WP_CORE_DIR/wp-content/db.php
+}
+
+install_test_suite() {
+	# portable in-place argument for both GNU sed and Mac OSX sed
+	if [[ $(uname -s) == 'Darwin' ]]; then
+		local ioption='-i .bak'
+	else
+		local ioption='-i'
+	fi
+
+	# set up testing suite if it doesn't yet exist
+	if [ ! -d $WP_TESTS_DIR ]; then
+		# set up testing suite
+		mkdir -p $WP_TESTS_DIR
+		svn co --quiet https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/includes/ $WP_TESTS_DIR/includes
+		svn co --quiet https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/data/ $WP_TESTS_DIR/data
+	fi
+
+	if [ ! -f wp-tests-config.php ]; then
+		download https://develop.svn.wordpress.org/${WP_TESTS_TAG}/wp-tests-config-sample.php "$WP_TESTS_DIR"/wp-tests-config.php
+		# remove all forward slashes in the end
+		WP_CORE_DIR=$(echo $WP_CORE_DIR | sed "s:/\+$::")
+		sed $ioption "s:dirname( __FILE__ ) . '/src/':'$WP_CORE_DIR/':" "$WP_TESTS_DIR"/wp-tests-config.php
+		sed $ioption "s/youremptytestdbnamehere/$DB_NAME/" "$WP_TESTS_DIR"/wp-tests-config.php
+		sed $ioption "s/yourusernamehere/$DB_USER/" "$WP_TESTS_DIR"/wp-tests-config.php
+		sed $ioption "s/yourpasswordhere/$DB_PASS/" "$WP_TESTS_DIR"/wp-tests-config.php
+		sed $ioption "s|localhost|${DB_HOST}|" "$WP_TESTS_DIR"/wp-tests-config.php
+	fi
+
+	# Modify the WP_UnitTestCase class to use the polyfilled version for PHPUnit cross-compatibility.
+	# This is a dirty "backport" of the polyfills used in WP 5.9 and might fail with future updates.
+	if [ ! -f "$WP_TESTS_DIR"/includes/abstract-testcase.php ]; then
+	  local testcase_file="$WP_TESTS_DIR"/includes/testcase.php
+	  sed $ioption 's/class WP_UnitTestCase extends PHPUnit_Framework_TestCase /class WP_UnitTestCase extends Yoast\\PHPUnitPolyfills\\TestCases\\TestCase /' "$testcase_file"
+    sed $ioption 's/setUpBeforeClass[(][)]/set_up_before_class()/g' "$testcase_file"
+    sed $ioption 's/tearDownAfterClass[(][)]/tear_down_after_class()/g' "$testcase_file"
+    sed $ioption 's/setUp[(][)]/set_up()/g' "$testcase_file"
+    sed $ioption 's/tearDown[(][)]/tear_down()/g' "$testcase_file"
+	fi
+
+}
+
+install_db() {
+
+	if [ ${SKIP_DB_CREATE} = "true" ]; then
+		return 0
+	fi
+
+	# parse DB_HOST for port or socket references
+	local PARTS=(${DB_HOST//\:/ })
+	local DB_HOSTNAME=${PARTS[0]};
+	local DB_SOCK_OR_PORT=${PARTS[1]};
+	local EXTRA=""
+
+	if ! [ -z $DB_HOSTNAME ] ; then
+		if [ $(echo $DB_SOCK_OR_PORT | grep -e '^[0-9]\{1,\}$') ]; then
+			EXTRA=" --host=$DB_HOSTNAME --port=$DB_SOCK_OR_PORT --protocol=tcp"
+		elif ! [ -z $DB_SOCK_OR_PORT ] ; then
+			EXTRA=" --socket=$DB_SOCK_OR_PORT"
+		elif ! [ -z $DB_HOSTNAME ] ; then
+			EXTRA=" --host=$DB_HOSTNAME --protocol=tcp"
+		fi
+	fi
+
+	# create database
+	mysqladmin create $DB_NAME --user="$DB_USER" --password="$DB_PASS"$EXTRA
+}
+
+install_wp
+install_test_suite
+install_db
diff --git a/composer.json b/composer.json
index c3405e4e..ee42659f 100644
--- a/composer.json
+++ b/composer.json
@@ -28,9 +28,11 @@
   "require-dev": {
     "dealerdirect/phpcodesniffer-composer-installer": "^v0.7",
     "matthiasmullie/minify": "^1.3",
+    "phpunit/phpunit": "^5|^7|^9",
     "squizlabs/php_codesniffer": "^3.7",
     "phpcompatibility/phpcompatibility-wp": "^2.1",
-    "wp-coding-standards/wpcs": "^2.3"
+    "wp-coding-standards/wpcs": "^2.3",
+    "yoast/phpunit-polyfills": "^1.0"
   },
   "scripts": {
     "post-install-cmd": [
@@ -63,6 +65,9 @@
     ],
     "lint-php": [
       "phpcs --standard=phpcs.xml -s"
+    ],
+    "test": [
+      "phpunit"
     ]
   },
   "config": {
diff --git a/inc/class-cachify-apc.php b/inc/class-cachify-apc.php
index 39761f4f..0e8c17c9 100644
--- a/inc/class-cachify-apc.php
+++ b/inc/class-cachify-apc.php
@@ -16,10 +16,9 @@ final class Cachify_APC {
 	/**
 	 * Availability check
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @return bool TRUE when installed
 	 *
-	 * @return  boolean  true/false  TRUE when installed
+	 * @since 2.0.7
 	 */
 	public static function is_available() {
 		return extension_loaded( 'apc' );
@@ -28,10 +27,9 @@ public static function is_available() {
 	/**
 	 * Caching method as string
 	 *
-	 * @since   2.1.2
-	 * @change  2.1.2
+	 * @return string Caching method
 	 *
-	 * @return  string  Caching method
+	 * @since 2.1.2
 	 */
 	public static function stringify_method() {
 		return 'APC';
@@ -40,13 +38,13 @@ public static function stringify_method() {
 	/**
 	 * Store item in cache
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param string $hash       Hash of the entry.
+	 * @param string $data       Content of the entry.
+	 * @param int    $lifetime   Lifetime of the entry.
+	 * @param bool   $sig_detail Show details in signature.
 	 *
-	 * @param   string  $hash        Hash of the entry.
-	 * @param   string  $data        Content of the entry.
-	 * @param   integer $lifetime    Lifetime of the entry.
-	 * @param   bool    $sig_detail  Show details in signature.
+	 * @since 2.0
+	 * @since 2.3.0 added $sigDetail parameter
 	 */
 	public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 		/* Do not store empty data. */
@@ -66,11 +64,11 @@ public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 	/**
 	 * Read item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash Hash of the entry.
 	 *
-	 * @param   string $hash  Hash of the entry.
-	 * @return  mixed         Content of the entry
+	 * @return mixed Content of the entry
+	 *
+	 * @since 2.0
 	 */
 	public static function get_item( $hash ) {
 		return ( function_exists( 'apc_exists' ) ? apc_exists( $hash ) : apc_fetch( $hash ) );
@@ -79,11 +77,10 @@ public static function get_item( $hash ) {
 	/**
 	 * Delete item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash Hash of the entry.
+	 * @param string $url  URL of the entry [optional].
 	 *
-	 * @param   string $hash  Hash of the entry.
-	 * @param   string $url   URL of the entry [optional].
+	 * @since 2.0
 	 */
 	public static function delete_item( $hash, $url = '' ) {
 		apc_delete( $hash );
@@ -92,8 +89,7 @@ public static function delete_item( $hash, $url = '' ) {
 	/**
 	 * Clear the cache
 	 *
-	 * @since   2.0.0
-	 * @change  2.0.7
+	 * @since 2.0
 	 */
 	public static function clear_cache() {
 		if ( ! self::is_available() ) {
@@ -106,8 +102,7 @@ public static function clear_cache() {
 	/**
 	 * Print the cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @since 2.0
 	 */
 	public static function print_cache() {
 		return;
@@ -116,10 +111,9 @@ public static function print_cache() {
 	/**
 	 * Get the cache size
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @return mixed Cache size
 	 *
-	 * @return  mixed  Cache size
+	 * @since 2.0
 	 */
 	public static function get_stats() {
 		/* Info */
@@ -136,11 +130,12 @@ public static function get_stats() {
 	/**
 	 * Generate signature
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param bool $detail Show details in signature.
+	 *
+	 * @return string Signature string
 	 *
-	 * @param   bool $detail  Show details in signature.
-	 * @return  string        Signature string
+	 * @since 2.0
+	 * @since 2.3.0 added $detail parameter
 	 */
 	private static function _cache_signature( $detail ) {
 		return sprintf(
diff --git a/inc/class-cachify-cli.php b/inc/class-cachify-cli.php
index 7317f191..4dc81d4c 100644
--- a/inc/class-cachify-cli.php
+++ b/inc/class-cachify-cli.php
@@ -16,11 +16,10 @@ final class Cachify_CLI {
 	/**
 	 * Flush Cache Callback
 	 *
-	 * @since   2.3.0
-	 * @change  2.3.0
-	 *
 	 * @param array $args the CLI arguments as array.
 	 * @param array $assoc_args the CLI arguments as associative array.
+	 *
+	 * @since 2.3.0
 	 */
 	public static function flush_cache( $args, $assoc_args ) {
 		// Set default arguments.
@@ -38,11 +37,10 @@ public static function flush_cache( $args, $assoc_args ) {
 	/**
 	 * Get cache size
 	 *
-	 * @since   2.3.0
-	 * @change  2.3.0
-	 *
 	 * @param array $args the CLI arguments as array.
 	 * @param array $assoc_args the CLI arguments as associative array.
+	 *
+	 * @since 2.3.0
 	 */
 	public static function get_cache_size( $args, $assoc_args ) {
 		// Set default arguments.
@@ -62,8 +60,7 @@ public static function get_cache_size( $args, $assoc_args ) {
 	/**
 	 * Register CLI Commands
 	 *
-	 * @since   2.3.0
-	 * @change  2.3.0
+	 * @since 2.3.0
 	 */
 	public static function add_commands() {
 		// Add flush command.
diff --git a/inc/class-cachify-db.php b/inc/class-cachify-db.php
index 86011462..003c0394 100644
--- a/inc/class-cachify-db.php
+++ b/inc/class-cachify-db.php
@@ -16,10 +16,9 @@ final class Cachify_DB {
 	/**
 	 * Availability check
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @return bool TRUE when installed
 	 *
-	 * @return  boolean  true/false  TRUE when installed
+	 * @since 2.0.7
 	 */
 	public static function is_available() {
 		return true;
@@ -28,10 +27,9 @@ public static function is_available() {
 	/**
 	 * Caching method as string
 	 *
-	 * @since   2.1.2
-	 * @change  2.1.2
+	 * @return string Caching method
 	 *
-	 * @return  string  Caching method
+	 * @since 2.1.2
 	 */
 	public static function stringify_method() {
 		return 'DB';
@@ -40,17 +38,17 @@ public static function stringify_method() {
 	/**
 	 * Store item in cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash     Hash of the entry.
+	 * @param string $data     Content of the entry.
+	 * @param int    $lifetime Lifetime of the entry.
 	 *
-	 * @param   string  $hash      Hash of the entry.
-	 * @param   string  $data      Content of the entry.
-	 * @param   integer $lifetime  Lifetime of the entry.
+	 * @since 2.0
 	 */
 	public static function store_item( $hash, $data, $lifetime ) {
 		/* Do not store empty data. */
 		if ( empty( $data ) ) {
 			trigger_error( __METHOD__ . ': Empty input.', E_USER_WARNING );
+
 			return;
 		}
 
@@ -73,11 +71,11 @@ public static function store_item( $hash, $data, $lifetime ) {
 	/**
 	 * Read item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash Hash of the entry.
+	 *
+	 * @return mixed Content of the entry
 	 *
-	 * @param   string $hash    Hash of the entry.
-	 * @return  mixed           Content of the entry
+	 * @since 2.0
 	 */
 	public static function get_item( $hash ) {
 		return get_transient( $hash );
@@ -86,11 +84,10 @@ public static function get_item( $hash ) {
 	/**
 	 * Delete item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash Hash of the entry.
+	 * @param string $url  URL of the entry [optional].
 	 *
-	 * @param   string $hash  Hash of the entry.
-	 * @param   string $url   URL of the entry [optional].
+	 * @since 2.0
 	 */
 	public static function delete_item( $hash, $url = '' ) {
 		delete_transient( $hash );
@@ -99,8 +96,7 @@ public static function delete_item( $hash, $url = '' ) {
 	/**
 	 * Clear the cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @since 2.0
 	 */
 	public static function clear_cache() {
 		/* Init */
@@ -113,11 +109,11 @@ public static function clear_cache() {
 	/**
 	 * Print the cache
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param bool  $sig_detail Show details in signature.
+	 * @param array $cache      Array of cache values.
 	 *
-	 * @param   bool  $sig_detail  Show details in signature.
-	 * @param   array $cache       Array of cache values.
+	 * @since 2.0
+	 * @since 2.3.0 added $sig_detail parameter
 	 */
 	public static function print_cache( $sig_detail, $cache ) {
 		/* No array? */
@@ -141,16 +137,16 @@ public static function print_cache( $sig_detail, $cache ) {
 	/**
 	 * Get the cache size
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @return int Column size
 	 *
-	 * @return  integer  Column size
+	 * @since 2.0
 	 */
 	public static function get_stats() {
 		/* Init */
 		global $wpdb;
 
 		/* Read */
+
 		return $wpdb->get_var(
 			'SELECT SUM( CHAR_LENGTH(option_value) ) FROM `' . $wpdb->options . "` WHERE `option_name` LIKE ('\_transient%.cachify')"
 		);
@@ -159,12 +155,13 @@ public static function get_stats() {
 	/**
 	 * Generate signature
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param bool  $detail Show details in signature.
+	 * @param array $meta   Content of metadata.
+	 *
+	 * @return string Signature string
 	 *
-	 * @param   bool  $detail  Show details in signature.
-	 * @param   array $meta    Content of metadata.
-	 * @return  string         Signature string
+	 * @since 2.0
+	 * @since 2.3.0 added $detail parameter
 	 */
 	private static function _cache_signature( $detail, $meta ) {
 		/* No array? */
@@ -210,10 +207,9 @@ private static function _cache_signature( $detail, $meta ) {
 	/**
 	 * Return query count
 	 *
-	 * @since   0.1
-	 * @change  2.0
+	 * @return int Number of queries
 	 *
-	 * @return  integer  Number of queries
+	 * @since 0.1
 	 */
 	private static function _page_queries() {
 		return $GLOBALS['wpdb']->num_queries;
@@ -222,10 +218,9 @@ private static function _page_queries() {
 	/**
 	 * Return execution time
 	 *
-	 * @since   0.1
-	 * @change  2.0
+	 * @return int Execution time in seconds
 	 *
-	 * @return  integer  Execution time in seconds
+	 * @since 0.1
 	 */
 	private static function _page_timer() {
 		return timer_stop( 0, 2 );
@@ -234,10 +229,9 @@ private static function _page_timer() {
 	/**
 	 * Return memory consumption
 	 *
-	 * @since   0.7
-	 * @change  2.0
+	 * @return string Formatted memory size
 	 *
-	 * @return  string  Formatted memory size
+	 * @since 0.7
 	 */
 	private static function _page_memory() {
 		return ( function_exists( 'memory_get_usage' ) ? size_format( memory_get_usage(), 2 ) : 0 );
diff --git a/inc/class-cachify-hdd.php b/inc/class-cachify-hdd.php
index f971baa6..8d521a1e 100644
--- a/inc/class-cachify-hdd.php
+++ b/inc/class-cachify-hdd.php
@@ -16,10 +16,9 @@ final class Cachify_HDD {
 	/**
 	 * Availability check
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @return bool TRUE when installed
 	 *
-	 * @return  boolean  true/false  TRUE when installed
+	 * @since 2.0.7
 	 */
 	public static function is_available() {
 		$option = get_option( 'permalink_structure' );
@@ -29,9 +28,9 @@ public static function is_available() {
 	/**
 	 * Returns if gzip file creation is enabled
 	 *
-	 * @since  2.4.0
-	 *
 	 * @return bool
+	 *
+	 * @since 2.4.0
 	 */
 	public static function is_gzip_enabled() {
 		/**
@@ -45,10 +44,9 @@ public static function is_gzip_enabled() {
 	/**
 	 * Caching method as string
 	 *
-	 * @since   2.1.2
-	 * @change  2.1.2
+	 * @return string Caching method
 	 *
-	 * @return  string  Caching method
+	 * @since 2.1.2
 	 */
 	public static function stringify_method() {
 		return 'HDD';
@@ -57,13 +55,13 @@ public static function stringify_method() {
 	/**
 	 * Store item in cache
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param string $hash       Hash  of the entry [ignored].
+	 * @param string $data       Content of the entry.
+	 * @param int    $lifetime   Lifetime of the entry [ignored].
+	 * @param bool   $sig_detail Show details in signature.
 	 *
-	 * @param   string  $hash        Hash  of the entry [ignored].
-	 * @param   string  $data        Content of the entry.
-	 * @param   integer $lifetime    Lifetime of the entry [ignored].
-	 * @param   bool    $sig_detail  Show details in signature.
+	 * @since 2.0
+	 * @since 2.3.0 added $sig_details parameter
 	 */
 	public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 		/* Do not store empty data. */
@@ -81,10 +79,9 @@ public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 	/**
 	 * Read item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @return bool True if cache is present.
 	 *
-	 * @return  boolean  True if cache is present.
+	 * @since 2.0
 	 */
 	public static function get_item() {
 		return is_readable(
@@ -95,11 +92,10 @@ public static function get_item() {
 	/**
 	 * Delete item from cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $hash Hash of the entry [ignored].
+	 * @param string $url  URL of the entry.
 	 *
-	 * @param   string $hash  Hash of the entry [ignored].
-	 * @param   string $url   URL of the entry.
+	 * @since 2.0
 	 */
 	public static function delete_item( $hash, $url ) {
 		self::_clear_dir(
@@ -110,8 +106,7 @@ public static function delete_item( $hash, $url ) {
 	/**
 	 * Clear the cache
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @since 2.0
 	 */
 	public static function clear_cache() {
 		self::_clear_dir(
@@ -123,8 +118,7 @@ public static function clear_cache() {
 	/**
 	 * Print the cache
 	 *
-	 * @since   2.0
-	 * @change  2.3
+	 * @since 2.0
 	 */
 	public static function print_cache() {
 		$filename = self::_file_html();
@@ -138,10 +132,9 @@ public static function print_cache() {
 	/**
 	 * Get the cache size
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @return int Directory size
 	 *
-	 * @return  integer  Directory size
+	 * @since 2.0
 	 */
 	public static function get_stats() {
 		return self::_dir_size( CACHIFY_CACHE_DIR );
@@ -150,11 +143,12 @@ public static function get_stats() {
 	/**
 	 * Generate signature
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param bool $detail Show details in signature.
 	 *
-	 * @param   bool $detail  Show details in signature.
-	 * @return  string        Signature string
+	 * @return string Signature string
+	 *
+	 * @since 2.0
+	 * @since 2.3.0 added $detail parameter
 	 */
 	private static function _cache_signature( $detail ) {
 		return sprintf(
@@ -171,10 +165,9 @@ private static function _cache_signature( $detail ) {
 	/**
 	 * Initialize caching process
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $data Cache content.
 	 *
-	 * @param   string $data  Cache content.
+	 * @since 2.0
 	 */
 	private static function _create_files( $data ) {
 		$file_path = self::_file_path();
@@ -200,11 +193,10 @@ private static function _create_files( $data ) {
 	/**
 	 * Create cache file
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $file Path to cache file.
+	 * @param string $data Cache content.
 	 *
-	 * @param   string $file  Path to cache file.
-	 * @param   string $data  Cache content.
+	 * @since 2.0
 	 */
 	private static function _create_file( $file, $data ) {
 		/* Writable? */
@@ -230,11 +222,10 @@ private static function _create_file( $file, $data ) {
 	/**
 	 * Clear directory
 	 *
-	 * @since   2.0
-	 * @change  2.0.5
+	 * @param string $dir       Directory path.
+	 * @param bool   $recursive true for clearing subdirectories as well.
 	 *
-	 * @param   string  $dir        Directory path.
-	 * @param   boolean $recursive  true for clearing subdirectories as well.
+	 * @since 2.0
 	 */
 	private static function _clear_dir( $dir, $recursive = false ) {
 		/* Remote training slash */
@@ -285,11 +276,11 @@ private static function _clear_dir( $dir, $recursive = false ) {
 	/**
 	 * Get directory size
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $dir Directory path.
+	 *
+	 * @return mixed Directory size
 	 *
-	 * @param   string $dir   Directory path.
-	 * @return  mixed         Directory size
+	 * @since 2.0
 	 */
 	public static function _dir_size( $dir = '.' ) {
 		/* Is directory? */
@@ -330,11 +321,11 @@ public static function _dir_size( $dir = '.' ) {
 	/**
 	 * Path to cache file
 	 *
-	 * @since   2.0
-	 * @change  2.0
+	 * @param string $path Request URI or permalink [optional].
 	 *
-	 * @param   string $path  Request URI or permalink [optional].
-	 * @return  string        Path to cache file
+	 * @return string Path to cache file
+	 *
+	 * @since 2.0
 	 */
 	private static function _file_path( $path = null ) {
 		$prefix = is_ssl() ? 'https-' : '';
@@ -362,11 +353,11 @@ private static function _file_path( $path = null ) {
 	/**
 	 * Path to HTML file
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param string $file_path File path [optional].
+	 *
+	 * @return string Path to HTML file
 	 *
-	 * @param   string $file_path   File path [optional].
-	 * @return  string              Path to HTML file
+	 * @since 2.0
 	 */
 	private static function _file_html( $file_path = '' ) {
 		return ( empty( $file_path ) ? self::_file_path() : $file_path ) . 'index.html';
@@ -375,11 +366,11 @@ private static function _file_html( $file_path = '' ) {
 	/**
 	 * Path to GZIP file
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @param string $file_path File path [optional].
 	 *
-	 * @param   string $file_path   File path [optional].
-	 * @return  string              Path to GZIP file
+	 * @return string Path to GZIP file
+	 *
+	 * @since 2.0
 	 */
 	private static function _file_gzip( $file_path = '' ) {
 		return ( empty( $file_path ) ? self::_file_path() : $file_path ) . 'index.html.gz';
@@ -447,6 +438,5 @@ private static function _user_can_delete( $file ) {
 		}
 
 		return true;
-
 	}
 }
diff --git a/inc/class-cachify-memcached.php b/inc/class-cachify-memcached.php
index 27cfb013..688fbe5f 100644
--- a/inc/class-cachify-memcached.php
+++ b/inc/class-cachify-memcached.php
@@ -16,18 +16,18 @@ final class Cachify_MEMCACHED {
 	/**
 	 * Memcached-Object
 	 *
-	 * @since  2.0.7
-	 * @var    object
+	 * @var object
+	 *
+	 * @since 2.0.7
 	 */
 	private static $_memcached;
 
 	/**
 	 * Availability check
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @return bool TRUE when installed
 	 *
-	 * @return  boolean  true/false  TRUE when installed
+	 * @since 2.0.7
 	 */
 	public static function is_available() {
 		return class_exists( 'Memcached' )
@@ -38,10 +38,9 @@ public static function is_available() {
 	/**
 	 * Caching method as string
 	 *
-	 * @since   2.1.2
-	 * @change  2.1.2
+	 * @return string Caching method
 	 *
-	 * @return  string  Caching method
+	 * @since 2.1.2
 	 */
 	public static function stringify_method() {
 		return 'Memcached';
@@ -50,13 +49,13 @@ public static function stringify_method() {
 	/**
 	 * Store item in cache
 	 *
-	 * @param   string  $hash       Hash of the entry [ignored].
-	 * @param   string  $data       Content of the entry.
-	 * @param   integer $lifetime   Lifetime of the entry.
-	 * @param   bool    $sig_detail  Show details in signature.
+	 * @param string $hash       Hash of the entry [ignored].
+	 * @param string $data       Content of the entry.
+	 * @param int    $lifetime   Lifetime of the entry.
+	 * @param bool   $sig_detail Show details in signature.
 	 *
-	 * @since   2.0.7
-	 * @change  2.3.0
+	 * @since 2.0.7
+	 * @since 2.3.0 added $sig_detail parameter
 	 */
 	public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 		/* Do not store empty data. */
@@ -81,11 +80,11 @@ public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
 	/**
 	 * Read item from cache
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @param string $hash Hash of the entry.
+	 *
+	 * @return mixed Content of the entry
 	 *
-	 * @param   string $hash  Hash of the entry.
-	 * @return  mixed         Content of the entry
+	 * @since 2.0.7
 	 */
 	public static function get_item( $hash ) {
 		/* Server connect */
@@ -102,11 +101,10 @@ public static function get_item( $hash ) {
 	/**
 	 * Delete item from cache
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @param string $hash Hash of the entry.
+	 * @param string $url  URL of the entry [optional].
 	 *
-	 * @param   string $hash  Hash of the entry.
-	 * @param   string $url   URL of the entry [optional].
+	 * @since 2.0.7
 	 */
 	public static function delete_item( $hash, $url = '' ) {
 		/* Server connect */
@@ -123,8 +121,7 @@ public static function delete_item( $hash, $url = '' ) {
 	/**
 	 * Clear the cache
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @since 2.0.7
 	 */
 	public static function clear_cache() {
 		/* Server connect */
@@ -143,8 +140,7 @@ public static function clear_cache() {
 	/**
 	 * Print the cache
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @since 2.0.7
 	 */
 	public static function print_cache() {
 		return;
@@ -153,10 +149,9 @@ public static function print_cache() {
 	/**
 	 * Get the cache size
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @return mixed Cache size
 	 *
-	 * @return  mixed  Cache size
+	 * @since 2.0.7
 	 */
 	public static function get_stats() {
 		/* Server connect */
@@ -186,11 +181,12 @@ public static function get_stats() {
 	/**
 	 * Generate signature
 	 *
-	 * @since   2.0.7
-	 * @change  2.3.0
+	 * @param bool $detail Show details in signature.
 	 *
-	 * @param   bool $detail  Show details in signature.
-	 * @return  string        Signature string
+	 * @return string Signature string
+	 *
+	 * @since 2.0.7
+	 * @since 2.3.0 added $detail parameter
 	 */
 	private static function _cache_signature( $detail ) {
 		return sprintf(
@@ -207,11 +203,11 @@ private static function _cache_signature( $detail ) {
 	/**
 	 * Path of cache file
 	 *
-	 * @since   2.0.7
-	 * @change  2.0.7
+	 * @param string $path Request URI or permalink [optional].
+	 *
+	 * @return string Path to cache file
 	 *
-	 * @param   string $path  Request URI or permalink [optional].
-	 * @return  string        Path to cache file
+	 * @since 2.0.7
 	 */
 	private static function _file_path( $path = null ) {
 		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.InputNotValidated
@@ -230,12 +226,11 @@ private static function _file_path( $path = null ) {
 	/**
 	 * Connect to Memcached server
 	 *
-	 * @since   2.0.7
-	 * @change  2.1.8
+	 * @hook  array  cachify_memcached_servers  Array with memcached servers
 	 *
-	 * @hook    array  cachify_memcached_servers  Array with memcached servers
+	 * @return bool TRUE on success
 	 *
-	 * @return  boolean  true/false  TRUE on success
+	 * @since 2.0.7
 	 */
 	private static function _connect_server() {
 		/* Not enabled? */
diff --git a/inc/class-cachify.php b/inc/class-cachify.php
index 7724bcec..9166a60d 100644
--- a/inc/class-cachify.php
+++ b/inc/class-cachify.php
@@ -16,32 +16,36 @@ final class Cachify {
 	/**
 	 * Plugin options
 	 *
-	 * @since  2.0
-	 * @var    array
+	 * @var array
+	 *
+	 * @since 2.0
 	 */
 	private static $options;
 
 	/**
 	 * Caching method
 	 *
-	 * @since  2.0
-	 * @var    object
+	 * @var object
+	 *
+	 * @since 2.0
 	 */
 	private static $method;
 
 	/**
-	 * Whether we are on an Nginx server or not.
+	 * Whether we are on a Nginx server or not.
+	 *
+	 * @var bool
 	 *
 	 * @since 2.2.5
-	 * @var   boolean
 	 */
 	private static $is_nginx;
 
 	/**
 	 * Method settings
 	 *
-	 * @since  2.0.9
-	 * @var    integer
+	 * @var int
+	 *
+	 * @since 2.0.9
 	 */
 	const METHOD_DB = 0;
 	const METHOD_APC = 1;
@@ -51,8 +55,9 @@ final class Cachify {
 	/**
 	 * Minify settings
 	 *
-	 * @since  2.0.9
-	 * @var    integer
+	 * @var int
+	 *
+	 * @since 2.0.9
 	 */
 	const MINIFY_DISABLED = 0;
 	const MINIFY_HTML_ONLY = 1;
@@ -61,7 +66,7 @@ final class Cachify {
 	/**
 	 * REST endpoints
 	 *
-	 * @var    string
+	 * @var string
 	 */
 	const REST_NAMESPACE = 'cachify/v1';
 	const REST_ROUTE_FLUSH = 'flush';
@@ -69,8 +74,7 @@ final class Cachify {
 	/**
 	 * Pseudo constructor
 	 *
-	 * @since   2.0.5
-	 * @change  2.0.5
+	 * @since 2.0.5
 	 */
 	public static function instance() {
 		new self();
@@ -79,10 +83,7 @@ public static function instance() {
 	/**
 	 * Constructor
 	 *
-	 * @since   1.0.0
-	 * @change  2.2.2
-	 *
-	 * @return  void
+	 * @since 1.0
 	 */
 	public function __construct() {
 		/* Set defaults */
@@ -173,8 +174,7 @@ public function __construct() {
 	/**
 	 * Deactivation hook
 	 *
-	 * @since   2.1.0
-	 * @change  2.1.0
+	 * @since 2.1.0
 	 */
 	public static function on_deactivation() {
 		/* Remove hdd cache cron when hdd is selected */
@@ -191,8 +191,7 @@ public static function on_deactivation() {
 	/**
 	 * Activation hook
 	 *
-	 * @since   1.0
-	 * @change  2.1.0
+	 * @since 1.0
 	 */
 	public static function on_activation() {
 		/* Multisite & Network */
@@ -217,10 +216,10 @@ public static function on_activation() {
 	/**
 	 * Plugin installation on new WPMS site.
 	 *
-	 * @since 1.0
-	 * @since 2.4 supports WP_Site argument
-	 *
 	 * @param int|WP_Site $new_site New site ID or object.
+	 *
+	 * @since 1.0
+	 * @since 2.4.0 supports WP_Site argument
 	 */
 	public static function install_later( $new_site ) {
 		/* No network plugin */
@@ -241,8 +240,7 @@ public static function install_later( $new_site ) {
 	/**
 	 * Actual installation of the options
 	 *
-	 * @since   1.0
-	 * @change  2.0
+	 * @since 1.0
 	 */
 	private static function _install_backend() {
 		add_option(
@@ -257,8 +255,7 @@ private static function _install_backend() {
 	/**
 	 * Uninstalling of the plugin per MU blog.
 	 *
-	 * @since   1.0
-	 * @change  2.1.0
+	 * @since 1.0
 	 */
 	public static function on_uninstall() {
 		/* Global */
@@ -288,10 +285,10 @@ public static function on_uninstall() {
 	/**
 	 * Uninstalling of the plugin for WPMS site.
 	 *
-	 * @since 1.0
-	 * @since 2.4 supports WP_Site argument
-	 *
 	 * @param int|WP_Site $old_site Old site ID or object.
+	 *
+	 * @since 1.0
+	 * @since 2.4.0 supports WP_Site argument
 	 */
 	public static function uninstall_later( $old_site ) {
 		/* No network plugin */
@@ -312,8 +309,7 @@ public static function uninstall_later( $old_site ) {
 	/**
 	 * Actual uninstalling of the plugin
 	 *
-	 * @since   1.0
-	 * @change  1.0
+	 * @since 1.0
 	 */
 	private static function _uninstall_backend() {
 		/* Option */
@@ -326,10 +322,9 @@ private static function _uninstall_backend() {
 	/**
 	 * Get IDs of installed blogs
 	 *
-	 * @since   1.0
-	 * @change  1.0
+	 * @return array Blog IDs
 	 *
-	 * @return  array  Blog IDs
+	 * @since 1.0
 	 */
 	private static function _get_blog_ids() {
 		/* Global */
@@ -380,8 +375,7 @@ public static function register_scripts() {
 	/**
 	 * Register the language file
 	 *
-	 * @since   2.1.3
-	 * @change  2.3.2
+	 * @since 2.1.3
 	 */
 	public static function register_textdomain() {
 		load_plugin_textdomain( 'cachify' );
@@ -390,8 +384,7 @@ public static function register_textdomain() {
 	/**
 	 * Set default options
 	 *
-	 * @since   2.0
-	 * @change  2.0.7
+	 * @since 2.0
 	 */
 	private static function _set_default_vars() {
 		/* Options */
@@ -418,10 +411,9 @@ private static function _set_default_vars() {
 	/**
 	 * Get options
 	 *
-	 * @since   2.0
-	 * @change  2.3.0
+	 * @return array Array of option values
 	 *
-	 * @return  array  Array of option values
+	 * @since 2.0
 	 */
 	private static function _get_options() {
 		return wp_parse_args(
@@ -445,7 +437,7 @@ private static function _get_options() {
 	 *
 	 * @since 1.0
 	 * @since 2.1.9
-	 * @since 2.4   Removed $data parameter and return value.
+	 * @since 2.4.0 Removed $data parameter and return value.
 	 */
 	public static function robots_txt() {
 		/* HDD only */
@@ -457,7 +449,7 @@ public static function robots_txt() {
 	/**
 	 * HDD Cache expiration cron action.
 	 *
-	 * @since 2.4
+	 * @since 2.4.0
 	 */
 	public static function run_hdd_cache_cron() {
 		Cachify_HDD::clear_cache();
@@ -470,7 +462,7 @@ public static function run_hdd_cache_cron() {
 	 *
 	 * @return array Array of non-default schedules with our tasks added.
 	 *
-	 * @since 2.4
+	 * @since 2.4.0
 	 */
 	public static function add_cron_cache_expiration( $schedules ) {
 		$schedules['cachify_cache_expire'] = array(
@@ -483,11 +475,11 @@ public static function add_cron_cache_expiration( $schedules ) {
 	/**
 	 * Add the action links
 	 *
-	 * @since   1.0
-	 * @change  1.0
+	 * @param array $data Initial array with action links.
 	 *
-	 * @param   array $data  Initial array with action links.
-	 * @return  array        Merged array with action links.
+	 * @return array Merged array with action links.
+	 *
+	 * @since 1.0
 	 */
 	public static function action_links( $data ) {
 		/* Permissions? */
@@ -515,12 +507,12 @@ public static function action_links( $data ) {
 	/**
 	 * Meta links of the plugin
 	 *
-	 * @since   0.5
-	 * @change  2.0.5
+	 * @param array  $input Initial array with meta links.
+	 * @param string $page  Current page.
+	 *
+	 * @return array Merged array with meta links.
 	 *
-	 * @param   array  $input  Initial array with meta links.
-	 * @param   string $page   Current page.
-	 * @return  array          Merged array with meta links.
+	 * @since 0.5
 	 */
 	public static function row_meta( $input, $page ) {
 		/* Permissions */
@@ -540,11 +532,11 @@ public static function row_meta( $input, $page ) {
 	/**
 	 * Add cache properties to dashboard
 	 *
-	 * @since   2.0.0
-	 * @change  2.2.2
+	 * @param array $items Initial array with dashboard items.
 	 *
-	 * @param   array $items  Initial array with dashboard items.
-	 * @return  array         Merged array with dashboard items.
+	 * @return array Merged array with dashboard items.
+	 *
+	 * @since 2.0.0
 	 */
 	public static function add_dashboard_count( $items = array() ) {
 		/* Skip */
@@ -597,10 +589,9 @@ public static function add_dashboard_count( $items = array() ) {
 	/**
 	 * Get the cache size
 	 *
-	 * @since   2.0.6
-	 * @change  2.0.6
+	 * @return int Cache size in bytes.
 	 *
-	 * @return  integer    Cache size in bytes.
+	 * @since 2.0.6
 	 */
 	public static function get_cache_size() {
 		$size = get_transient( 'cachify_cache_size' );
@@ -627,13 +618,13 @@ public static function get_cache_size() {
 	/**
 	 * Add flush icon to admin bar menu
 	 *
-	 * @since   1.2
-	 * @change  2.2.2
-	 * @change  2.4.0 Adjust icon for flush request via AJAX
+	 * @hook    mixed cachify_user_can_flush_cache
 	 *
-	 * @hook    mixed   cachify_user_can_flush_cache
+	 * @param object $wp_admin_bar Object of menu items.
 	 *
-	 * @param   object $wp_admin_bar  Object of menu items.
+	 * @since 1.2
+	 * @since 2.2.2
+	 * @since 2.4.0 Adjust icon for flush request via AJAX
 	 */
 	public static function add_flush_icon( $wp_admin_bar ) {
 		/* Quit */
@@ -675,9 +666,9 @@ public static function add_flush_icon( $wp_admin_bar ) {
 	/**
 	 * Returns the dashicon class for the success state in admin bar flush button
 	 *
-	 * @since  2.4.0
-	 *
 	 * @return string
+	 *
+	 * @since 2.4.0
 	 */
 	public static function get_dashicon_success_class() {
 		global $wp_version;
@@ -691,11 +682,11 @@ public static function get_dashicon_success_class() {
 	/**
 	 * Add a script to query the REST endpoint and animate the flush icon in admin bar menu
 	 *
-	 * @since   2.4.0
+	 * @hook  mixed  cachify_user_can_flush_cache ?
 	 *
-	 * @hook    mixed   cachify_user_can_flush_cache ?
+	 * @param object $wp_admin_bar Object of menu items.
 	 *
-	 * @param   object $wp_admin_bar  Object of menu items.
+	 * @since 2.4.0
 	 */
 	public static function add_flush_icon_script( $wp_admin_bar ) {
 		/* Quit */
@@ -724,7 +715,7 @@ public static function add_flush_icon_script( $wp_admin_bar ) {
 	/**
 	 * Registers an REST endpoint for the flush operation
 	 *
-	 * @change 2.4.0
+	 * @since 2.4.0
 	 */
 	public static function add_flush_rest_endpoint() {
 		register_rest_route(
@@ -747,9 +738,9 @@ public static function add_flush_rest_endpoint() {
 	/**
 	 * Check if user can manage options
 	 *
-	 * @since   2.4.0
+	 * @return bool
 	 *
-	 * @return  bool
+	 * @since 2.4.0
 	 */
 	public static function user_can_manage_options() {
 		return current_user_can( 'manage_options' );
@@ -758,13 +749,13 @@ public static function user_can_manage_options() {
 	/**
 	 * Process plugin's meta actions
 	 *
-	 * @since   0.5
-	 * @change  2.2.2
-	 * @change  2.4.0  Extract cache flushing to own method and always redirect to referer with new value for `_cachify` param.
-	 *
 	 * @hook    mixed  cachify_user_can_flush_cache
 	 *
-	 * @param   array $data  Metadata of the plugin.
+	 * @param array $data Metadata of the plugin.
+	 *
+	 * @since 0.5
+	 * @since 2.2.2
+	 * @since 2.4.0  Extract cache flushing to own method and always redirect to referer with new value for `_cachify` param.
 	 */
 	public static function process_flush_request( $data ) {
 		/* Skip if not a flush request */
@@ -861,10 +852,10 @@ public static function flush_cache() {
 	/**
 	 * Notice after successful flushing of the cache
 	 *
-	 * @since   1.2
-	 * @change  2.2.2
+	 * @hook  mixed  cachify_user_can_flush_cache
 	 *
-	 * @hook    mixed  cachify_user_can_flush_cache
+	 * @since 1.2
+	 * @since 2.2.2
 	 */
 	public static function flush_notice() {
 		/* No admin */
@@ -881,12 +872,12 @@ public static function flush_notice() {
 	/**
 	 * Remove page from cache or flush on comment edit
 	 *
-	 * @since   0.1.0
-	 * @change  2.1.2
+	 * @param int $id Comment ID.
 	 *
-	 * @param   integer $id  Comment ID.
+	 * @since 0.1.0
+	 * @since 2.1.2
 	 *
-	 * @deprecated 2.4 Use comment_edit($id, $comment) instead.
+	 * @deprecated 2.4.0 Use comment_edit($id, $comment) instead.
 	 */
 	public static function edit_comment( $id ) {
 		self::comment_edit( $id, array( 'comment_approved' => 1 ) );
@@ -895,10 +886,10 @@ public static function edit_comment( $id ) {
 	/**
 	 * Remove page from cache or flush on comment edit.
 	 *
-	 * @since 2.4 Replacement for edit_comment($id) with additional comment parameter.
-	 *
 	 * @param integer $id      Comment ID.
 	 * @param array   $comment Comment data.
+	 *
+	 * @since 2.4.0 Replacement for edit_comment($id) with additional comment parameter.
 	 */
 	public static function comment_edit( $id, $comment ) {
 		$approved = (int) $comment['comment_approved'];
@@ -918,15 +909,14 @@ public static function comment_edit( $id, $comment ) {
 	/**
 	 * Remove page from cache or flush on new comment
 	 *
-	 * @since   0.1.0
-	 * @change  2.1.2
-	 *
 	 * @param mixed $approved Comment status.
 	 * @param array $comment  Array of properties.
 	 *
 	 * @return mixed Comment status.
 	 *
-	 * @deprecated 2.4 Use new_comment($id, $approved) instead.
+	 * @since 0.1
+	 * @since 2.1.2
+	 * @since 2.4.0 Replacement for edit_comment($id) with additional comment parameter.
 	 */
 	public static function pre_comment( $approved, $comment ) {
 		self::new_comment( $comment['comment_ID'], $approved );
@@ -937,12 +927,12 @@ public static function pre_comment( $approved, $comment ) {
 	/**
 	 * Remove page from cache or flush on new comment
 	 *
-	 * @since 0.1.0
-	 * @since 2.1.2
-	 * @since 2.4 Renamed with ID parameter instead of comment array.
-	 *
 	 * @param integer|string $id       Comment ID.
 	 * @param integer|string $approved Comment status.
+	 *
+	 * @since 0.1.0
+	 * @since 2.1.2
+	 * @since 2.4.0 Renamed with ID parameter instead of comment array.
 	 */
 	public static function new_comment( $id, $approved ) {
 		/* Approved comment? */
@@ -958,14 +948,14 @@ public static function new_comment( $id, $approved ) {
 	/**
 	 * Remove page from cache or flush on comment edit
 	 *
-	 * @since   0.1
-	 * @change  2.1.2
+	 * @param string $new_status New status.
+	 * @param string $old_status Old status.
+	 * @param object $comment    The comment.
 	 *
-	 * @param   string $new_status  New status.
-	 * @param   string $old_status  Old status.
-	 * @param   object $comment     The comment.
+	 * @since 0.1
+	 * @since 2.1.2
 	 *
-	 * @deprecated 2.4 Use comment_status($new_status, $old_status, $comment) instead.
+	 * @deprecated 2.4.0 Use comment_status($new_status, $old_status, $comment) instead.
 	 */
 	public static function touch_comment( $new_status, $old_status, $comment ) {
 		self::comment_status( $new_status, $old_status, $comment );
@@ -974,13 +964,13 @@ public static function touch_comment( $new_status, $old_status, $comment ) {
 	/**
 	 * Remove page from cache or flush on comment edit.
 	 *
-	 * @since 0.1
-	 * @since 2.1.2
-	 * @since 2.4 Renamed from touch_comment().
-	 *
 	 * @param string     $new_status New status.
 	 * @param string     $old_status Old status.
 	 * @param WP_Comment $comment    The comment.
+	 *
+	 * @since 0.1
+	 * @since 2.1.2
+	 * @since 2.4.0 Renamed from touch_comment().
 	 */
 	public static function comment_status( $new_status, $old_status, $comment ) {
 		if ( 'approved' === $old_status || 'approved' === $new_status ) {
@@ -995,8 +985,8 @@ public static function comment_status( $new_status, $old_status, $comment ) {
 	/**
 	 * Generate publish hook for custom post types
 	 *
-	 * @since   2.1.7  Make the function public
-	 * @since   2.0.3
+	 * @since 2.0.3
+	 * @since 2.1.7  Make the function public
 	 *
 	 * @deprecated no longer used since 2.4
 	 */
@@ -1023,11 +1013,10 @@ public static function register_publish_hooks() {
 	/**
 	 * Removes the post type cache on post updates
 	 *
-	 * @since   2.0.3
-	 * @change  2.3.0
+	 * @param int    $post_id Post ID.
+	 * @param object $post    Post object.
 	 *
-	 * @param   integer $post_id  Post ID.
-	 * @param   object  $post     Post object.
+	 * @since 2.0.3
 	 *
 	 * @deprecated no longer used since 2.4
 	 */
@@ -1058,11 +1047,11 @@ public static function publish_post_types( $post_id, $post ) {
 	/**
 	 * Removes the post type cache if saved or updated
 	 *
+	 * @param int $id Post ID.
+	 *
 	 * @since 2.0.3
 	 * @since 2.1.7 Make the function public.
-	 * @since 2.4   Renamed to save_update_trash_post with $id parameter.
-	 *
-	 * @param integer $id Post ID.
+	 * @since 2.4.0 Renamed to save_update_trash_post with $id parameter.
 	 */
 	public static function save_update_trash_post( $id ) {
 		$status = get_post_status( $id );
@@ -1076,12 +1065,12 @@ public static function save_update_trash_post( $id ) {
 	/**
 	 * Removes the post type cache before an existing post type is updated in the db
 	 *
+	 * @param int   $id   Post ID.
+	 * @param array $data Post data.
+	 *
 	 * @since 2.0.3
 	 * @since 2.3.0
-	 * @since 2.4   Renamed to post_update.
-	 *
-	 * @param integer $id   Post ID.
-	 * @param array   $data Post data.
+	 * @since 2.4.0 Renamed to post_update.
 	 */
 	public static function post_update( $id, $data ) {
 		$new_status = $data['post_status'];
@@ -1096,9 +1085,9 @@ public static function post_update( $id, $data ) {
 	/**
 	 * Clear cache when any post type has been created or updated
 	 *
-	 * @since 2.4
+	 * @param int|WP_Post $post Post ID or object.
 	 *
-	 * @param integer|WP_Post $post Post ID or object.
+	 * @since 2.4.0
 	 */
 	public static function flush_cache_for_posts( $post ) {
 		if ( is_int( $post ) ) {
@@ -1125,9 +1114,9 @@ public static function flush_cache_for_posts( $post ) {
 	/**
 	 * Flush post cache on WooCommerce stock changes.
 	 *
-	 * @since 2.4
-	 *
 	 * @param int|WC_Product $product Product ID or object.
+	 *
+	 * @since 2.4.0
 	 */
 	public static function flush_woocommerce( $product ) {
 		if ( is_int( $product ) ) {
@@ -1142,10 +1131,9 @@ public static function flush_woocommerce( $product ) {
 	/**
 	 * Removes a page (id) from cache
 	 *
-	 * @since   2.0.3
-	 * @change  2.1.3
+	 * @param int $post_id Post ID.
 	 *
-	 * @param   integer $post_id  Post ID.
+	 * @since 2.0.3
 	 */
 	public static function remove_page_cache_by_post_id( $post_id ) {
 		$post_id = (int) $post_id;
@@ -1159,10 +1147,9 @@ public static function remove_page_cache_by_post_id( $post_id ) {
 	/**
 	 * Removes a page url from cache
 	 *
-	 * @since   0.1
-	 * @change  2.1.3
+	 * @param string $url Page URL.
 	 *
-	 * @param  string $url  Page URL.
+	 * @since 0.1
 	 */
 	public static function remove_page_cache_by_url( $url ) {
 		$url = (string) $url;
@@ -1183,10 +1170,9 @@ public static function remove_page_cache_by_url( $url ) {
 	/**
 	 * Get cache validity
 	 *
-	 * @since   2.0.0
-	 * @change  2.1.7
+	 * @return int Validity period in seconds.
 	 *
-	 * @return  integer    Validity period in seconds.
+	 * @since 2.0.0
 	 */
 	private static function _cache_expires() {
 		return HOUR_IN_SECONDS * self::$options['cache_expires'];
@@ -1195,9 +1181,9 @@ private static function _cache_expires() {
 	/**
 	 * Determine if cache details should be printed in signature
 	 *
-	 * @since   2.3.0
+	 * @return bool Show details in signature.
 	 *
-	 * @return  bool  Show details in signature.
+	 * @since 2.3.0
 	 */
 	private static function _signature_details() {
 		return 1 === self::$options['sig_detail'];
@@ -1206,12 +1192,12 @@ private static function _signature_details() {
 	/**
 	 * Get hash value for caching
 	 *
-	 * @since   0.1
-	 * @change  2.0
-	 * @change  2.4.0 Fix issue with port in URL.
+	 * @param string $url URL to hash [optional].
 	 *
-	 * @param   string $url  URL to hash [optional].
-	 * @return  string       Cachify hash value.
+	 * @return string Cachify hash value.
+	 *
+	 * @since 0.1
+	 * @since 2.0
 	 */
 	private static function _cache_hash( $url = '' ) {
 		$prefix = is_ssl() ? 'https-' : '';
@@ -1229,11 +1215,12 @@ private static function _cache_hash( $url = '' ) {
 	/**
 	 * Split by comma
 	 *
-	 * @since   0.9.1
-	 * @change  1.0
+	 * @param string $input String to split.
+	 *
+	 * @return array Splitted values.
 	 *
-	 * @param   string $input  String to split.
-	 * @return  array          Splitted values.
+	 * @since 0.9.1
+	 * @since 1.0
 	 */
 	private static function _preg_split( $input ) {
 		return (array) preg_split( '/,/', $input, -1, PREG_SPLIT_NO_EMPTY );
@@ -1242,10 +1229,9 @@ private static function _preg_split( $input ) {
 	/**
 	 * Check for index page
 	 *
-	 * @since   0.6
-	 * @change  1.0
+	 * @return bool TRUE if index
 	 *
-	 * @return  boolean  TRUE if index
+	 * @since 0.6
 	 */
 	private static function _is_index() {
 		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized, WordPress.Security.ValidatedSanitizedInput.InputNotValidated
@@ -1255,10 +1241,9 @@ private static function _is_index() {
 	/**
 	 * Check for mobile devices
 	 *
-	 * @since   0.9.1
-	 * @change  2.3.0
+	 * @return bool TRUE if mobile
 	 *
-	 * @return  boolean  TRUE if mobile
+	 * @since 0.9.1
 	 */
 	private static function _is_mobile() {
 		$templatedir = get_template_directory();
@@ -1268,10 +1253,9 @@ private static function _is_mobile() {
 	/**
 	 * Check if user is logged in or marked
 	 *
-	 * @since   2.0.0
-	 * @change  2.0.5
+	 * @return bool TRUE on "marked" users
 	 *
-	 * @return  boolean  $diff  TRUE on "marked" users
+	 * @since 2.0.0
 	 */
 	private static function _is_logged_in() {
 		/* Logged in */
@@ -1297,7 +1281,7 @@ private static function _is_logged_in() {
 	/**
 	 * Register all hooks to flush the total cache
 	 *
-	 * @since   2.4.0
+	 * @since 2.4.0
 	 */
 	public static function register_flush_cache_hooks() {
 		/* Define all default flush cache hooks */
@@ -1310,8 +1294,9 @@ public static function register_flush_cache_hooks() {
 			'create_term' => 10,
 			'delete_term' => 10,
 			'edit_terms' => 10,
-			'delete_user' => 10,
+			'user_register' => 10,
 			'edit_user_profile_update' => 10,
+			'delete_user' => 10,
 			/* third party */
 			'autoptimize_action_cachepurged' => 10,
 		);
@@ -1328,13 +1313,11 @@ public static function register_flush_cache_hooks() {
 	/**
 	 * Define exclusions for caching
 	 *
-	 * @since   0.2
-	 * @change  2.3.0
-	 * @change  2.4.0 Add check for sitemap feature and skip cache for other request methods than GET.
+	 * @hook bool cachify_skip_cache
 	 *
-	 * @return  boolean              TRUE on exclusion
+	 * @return bool TRUE on exclusion
 	 *
-	 * @hook    boolean  cachify_skip_cache
+	 * @since 0.2
 	 */
 	private static function _skip_cache() {
 
@@ -1403,19 +1386,26 @@ private static function _skip_cache() {
 			return true;
 		}
 
+		/* Content Negotiation */
+
+		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.MissingUnslash, WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
+		if ( isset( $_SERVER['HTTP_ACCEPT'] ) && false === strpos( $_SERVER['HTTP_ACCEPT'], 'text/html' ) ) {
+			return true;
+		}
+
 		return false;
 	}
 
 	/**
 	 * Minify HTML code
 	 *
-	 * @since   0.9.2
-	 * @change  2.0.9
+	 * @hook  array cachify_minify_ignore_tags
+	 *
+	 * @param string $data Original HTML code.
 	 *
-	 * @param   string $data  Original HTML code.
-	 * @return  string        Minified code
+	 * @return string Minified code
 	 *
-	 * @hook    array   cachify_minify_ignore_tags
+	 * @since 0.9.2
 	 */
 	private static function _minify_cache( $data ) {
 		/* Disabled? */
@@ -1474,11 +1464,10 @@ private static function _minify_cache( $data ) {
 	/**
 	 * Flush total cache
 	 *
-	 * @since   0.1
-	 * @change  2.0
-	 * @change  2.4.0 Do not flush cache for post revisions.
+	 * @param bool $clear_all_methods Flush all caching methods (default: FALSE).
 	 *
-	 * @param bool $clear_all_methods  Flush all caching methods (default: FALSE).
+	 * @since 0.1
+	 * @since 2.0
 	 */
 	public static function flush_total_cache( $clear_all_methods = false ) {
 		// We do not need to flush the cache for saved post revisions.
@@ -1514,11 +1503,12 @@ public static function flush_total_cache( $clear_all_methods = false ) {
 	/**
 	 * Assign the cache
 	 *
-	 * @since   0.1
-	 * @change  2.0
+	 * @param string $data Content of the page.
+	 *
+	 * @return string Content of the page.
 	 *
-	 * @param   string $data  Content of the page.
-	 * @return  string        Content of the page.
+	 * @since 0.1
+	 * @since 2.0
 	 */
 	public static function set_cache( $data ) {
 		/* Empty? */
@@ -1529,13 +1519,13 @@ public static function set_cache( $data ) {
 		/**
 		 * Filters whether the buffered data should actually be cached
 		 *
-		 * @since 2.3
-		 *
 		 * @param bool   $should_cache  Whether the data should be cached.
 		 * @param string $data          The actual data.
 		 * @param object $method        Instance of the selected caching method.
 		 * @param string $cache_hash    The cache hash.
 		 * @param int    $cache_expires Cache validity period.
+		 *
+		 * @since 2.3.0
 		 */
 		$should_cache = apply_filters(
 			'cachify_store_item',
@@ -1551,12 +1541,12 @@ public static function set_cache( $data ) {
 			/**
 			 * Filters the buffered data itself
 			 *
-			 * @since 2.4
-			 *
 			 * @param string $data          The actual data.
 			 * @param object $method        Instance of the selected caching method.
 			 * @param string $cache_hash    The cache hash.
 			 * @param int    $cache_expires Cache validity period.
+			 *
+			 * @since 2.4.0
 			 */
 			$data = apply_filters( 'cachify_modify_output', $data, self::$method, self::_cache_hash(), self::_cache_expires() );
 
@@ -1578,8 +1568,7 @@ public static function set_cache( $data ) {
 	/**
 	 * Manage the cache.
 	 *
-	 * @since   0.1
-	 * @change  2.3
+	 * @since 0.1
 	 */
 	public static function manage_cache() {
 		/* No caching? */
@@ -1616,10 +1605,9 @@ public static function manage_cache() {
 	/**
 	 * Register CSS
 	 *
-	 * @since   1.0
-	 * @change  2.3.0
+	 * @param string $hook Current hook.
 	 *
-	 * @param   string $hook  Current hook.
+	 * @since 1.0
 	 */
 	public static function add_admin_resources( $hook ) {
 		/* Hooks check */
@@ -1664,8 +1652,7 @@ public static function admin_dashboard_dark_mode_styles() {
 	/**
 	 * Add options page
 	 *
-	 * @since   1.0
-	 * @change  2.2.2
+	 * @since 1.0
 	 */
 	public static function add_page() {
 		add_options_page(
@@ -1683,10 +1670,9 @@ public static function add_page() {
 	/**
 	 * Available caching methods
 	 *
-	 * @since  2.0.0
-	 * @change 2.1.3
+	 * @return array Array of actually available methods.
 	 *
-	 * @return array           Array of actually available methods.
+	 * @since 2.0
 	 */
 	private static function _method_select() {
 		/* Defaults */
@@ -1718,10 +1704,9 @@ private static function _method_select() {
 	/**
 	 * Minify cache dropdown
 	 *
-	 * @since   2.1.3
-	 * @change  2.1.3
+	 * @return array Key => value array
 	 *
-	 * @return  array    Key => value array
+	 * @since 2.1.3
 	 */
 	private static function _minify_select() {
 		return array(
@@ -1734,8 +1719,7 @@ private static function _minify_select() {
 	/**
 	 * Register settings
 	 *
-	 * @since   1.0
-	 * @change  1.0
+	 * @since 1.0
 	 */
 	public static function register_settings() {
 		register_setting(
@@ -1751,11 +1735,12 @@ public static function register_settings() {
 	/**
 	 * Validate options
 	 *
-	 * @since   1.0.0
-	 * @change  2.1.3
+	 * @param array $data Array of form values.
+	 *
+	 * @return array Array of validated values.
 	 *
-	 * @param   array $data  Array of form values.
-	 * @return  array        Array of validated values.
+	 * @since 1.0
+	 * @since 2.1.3
 	 */
 	public static function validate_options( $data ) {
 		/* Empty data? */
@@ -1793,8 +1778,7 @@ public static function validate_options( $data ) {
 	/**
 	 * Display options page
 	 *
-	 * @since   1.0
-	 * @change  2.3.0
+	 * @since 1.0
 	 */
 	public static function options_page() {
 		$options = self::_get_options();
@@ -1843,11 +1827,11 @@ public static function options_page() {
 	/**
 	 * Return an array with all settings tabs applicable in context of current plugin options.
 	 *
-	 * @since   2.3.0
-	 * @change  2.3.0
-	 *
 	 * @param array $options the options.
+	 *
 	 * @return array
+	 *
+	 * @since 2.3.0
 	 */
 	private static function _get_tabs( $options ) {
 		/* Settings tab is always present */
diff --git a/inc/setup/cachify.hdd.htaccess.php b/inc/setup/cachify.hdd.htaccess.php
index 1f64549a..4ab3335c 100644
--- a/inc/setup/cachify.hdd.htaccess.php
+++ b/inc/setup/cachify.hdd.htaccess.php
@@ -26,6 +26,7 @@
   RewriteRule .* - [E=CACHIFY_DIR:/]
 {{GZIP}}
   # Main Rules
+  RewriteCond %{HTTP_ACCEPT} .*text/html.*
   RewriteCond %{REQUEST_METHOD} GET
   RewriteCond %{QUERY_STRING} =""
   RewriteCond %{REQUEST_URI} !^/(wp-admin|wp-content/cache)/.*
diff --git a/inc/setup/cachify.hdd.nginx.php b/inc/setup/cachify.hdd.nginx.php
index 7aaf1f04..38b001c6 100644
--- a/inc/setup/cachify.hdd.nginx.php
+++ b/inc/setup/cachify.hdd.nginx.php
@@ -24,6 +24,9 @@
   if ( $query_string ) {
 	return 405;
   }
+  if ( $http_accept !~* "text/html" ) {
+	return 405;
+  }
   if ( $request_method = POST ) {
 	return 405;
   }
diff --git a/inc/setup/cachify.memcached.nginx.php b/inc/setup/cachify.memcached.nginx.php
index 2b737b8c..ca3b1518 100644
--- a/inc/setup/cachify.memcached.nginx.php
+++ b/inc/setup/cachify.memcached.nginx.php
@@ -26,6 +26,11 @@
   if ( $query_string ) {
 	return 405;
   }
+
+  if ( $http_accept !~* "text/html" ) {
+	return 405;
+  }
+
   if ( $request_method = POST ) {
 	return 405;
   }
diff --git a/phpunit.xml b/phpunit.xml
new file mode 100644
index 00000000..b16e11ae
--- /dev/null
+++ b/phpunit.xml
@@ -0,0 +1,25 @@
+<?xml version="1.0"?>
+<phpunit bootstrap="tests/bootstrap.php"
+         backupGlobals="false"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true">
+    <coverage processUncoveredFiles="false">
+        <include>
+            <file>cachify.php</file>
+            <directory suffix=".php">inc</directory>
+        </include>
+        <report>
+            <clover outputFile="phpunit.coverage.xml"/>
+        </report>
+    </coverage>
+    <testsuites>
+        <testsuite name="default">
+            <directory prefix="test-" suffix=".php">./tests/</directory>
+        </testsuite>
+    </testsuites>
+    <logging>
+        <junit outputFile="phpunit.report.xml"/>
+    </logging>
+</phpunit>
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
new file mode 100644
index 00000000..2a307196
--- /dev/null
+++ b/tests/bootstrap.php
@@ -0,0 +1,30 @@
+<?php
+/**
+ * PHPUnit bootstrap file
+ */
+
+$_tests_dir = getenv( 'WP_TESTS_DIR' );
+
+if ( ! $_tests_dir ) {
+	$_tests_dir = rtrim( sys_get_temp_dir(), '/\\' ) . '/wordpress-tests-lib';
+}
+
+if ( ! file_exists( $_tests_dir . '/includes/functions.php' ) ) {
+	echo "Could not find $_tests_dir/includes/functions.php, have you run bin/install-wp-tests.sh ?";
+	exit( 1 );
+}
+
+// Give access to tests_add_filter() function.
+require_once $_tests_dir . '/includes/functions.php';
+
+/**
+ * Manually load the plugin being tested.
+ */
+function _manually_load_plugin() {
+	require dirname( dirname( __FILE__ ) ) . '/cachify.php';
+}
+
+tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );
+
+// Start up the WP testing environment.
+require $_tests_dir . '/includes/bootstrap.php';
diff --git a/tests/test-cachify-apc.php b/tests/test-cachify-apc.php
new file mode 100644
index 00000000..414fcd07
--- /dev/null
+++ b/tests/test-cachify-apc.php
@@ -0,0 +1,31 @@
+<?php
+/**
+ * Cachify APC tests.
+ *
+ * @package Cachify
+ */
+
+/**
+ * Class Test_Cachify_APC.
+ *
+ * Tests for the Cachify APC backend.
+ */
+class Test_Cachify_APC extends WP_UnitTestCase {
+	/**
+	 * Test backend availability.
+	 */
+	public function test_is_available() {
+		/*
+		 * This test is a little tricky, because we need the PHP APC extension loaded.
+		 * Let's just assume the equivalence between availability of the extension and the backend here.
+		 */
+		self::assertEquals( extension_loaded( 'apc' ), Cachify_APC::is_available() );
+	}
+
+	/**
+	 * Test backend availability.
+	 */
+	public function test_stringify_method() {
+		self::assertEquals( 'APC', Cachify_APC::stringify_method() );
+	}
+}
diff --git a/tests/test-cachify-db.php b/tests/test-cachify-db.php
new file mode 100644
index 00000000..815555eb
--- /dev/null
+++ b/tests/test-cachify-db.php
@@ -0,0 +1,65 @@
+<?php
+/**
+ * Cachify DB tests.
+ *
+ * @package Cachify
+ */
+
+/**
+ * Class Test_Cachify_DB.
+ *
+ * Tests for the Cachify database backend.
+ */
+class Test_Cachify_DB extends WP_UnitTestCase {
+	/**
+	 * Test backend availability.
+	 */
+	public function test_is_available() {
+		self::assertTrue( Cachify_DB::is_available() );
+	}
+
+	/**
+	 * Test backend availability.
+	 */
+	public function test_stringify_method() {
+		self::assertEquals( 'DB', Cachify_DB::stringify_method() );
+	}
+
+	/**
+	 * Test the actual caching.
+	 */
+	public function test_caching() {
+		self::assertFalse( Cachify_DB::get_item( '965b4abf2414e45036ab90c9d3f8dbc7' ) );
+
+		Cachify_DB::store_item(
+			'965b4abf2414e45036ab90c9d3f8dbc7',
+			'<html><head><title>Test Me</title></head><body><p>Test Content.</p></body></html>',
+			3600
+		);
+
+		$cached = Cachify_DB::get_item( '965b4abf2414e45036ab90c9d3f8dbc7' );
+		self::assertIsArray( $cached, 'item was not stored' );
+		self::assertEquals(
+			'<html><head><title>Test Me</title></head><body><p>Test Content.</p></body></html>',
+			$cached['data'],
+			'unexpected data in cache'
+		);
+		self::assertIsInt( $cached['meta']['queries'], 'number of queries not filled' );
+		self::assertIsString( $cached['meta']['timer'], 'timing not filled' );
+		self::assertIsString( $cached['meta']['memory'], 'memory not filled' );
+		self::assertIsInt( $cached['meta']['time'], 'time not filled' );
+
+		// Another item.
+		Cachify_DB::store_item(
+			'ef7e4a0540f6cde19e6eb658c69b0064',
+			'<html><head><title>Test 2</title></head><body><p>Test Content #2.</p></body></html>',
+			3600
+		);
+		self::assertIsArray( Cachify_DB::get_item( 'ef7e4a0540f6cde19e6eb658c69b0064' ), 'second item was not stored' );
+
+		// Delete the first item.
+		Cachify_DB::delete_item( '965b4abf2414e45036ab90c9d3f8dbc7' );
+		self::assertFalse( Cachify_DB::get_item( '965b4abf2414e45036ab90c9d3f8dbc7' ), 'first item was not deleted' );
+		self::assertIsArray( Cachify_DB::get_item( 'ef7e4a0540f6cde19e6eb658c69b0064' ), 'second item should still be present' );
+	}
+}
diff --git a/tests/test-cachify-hdd.php b/tests/test-cachify-hdd.php
new file mode 100644
index 00000000..80bf3656
--- /dev/null
+++ b/tests/test-cachify-hdd.php
@@ -0,0 +1,105 @@
+<?php
+/**
+ * Cachify HDD tests.
+ *
+ * @package Cachify
+ */
+
+/**
+ * Class Test_Cachify_HDD.
+ *
+ * Tests for the Cachify harddisk backend.
+ */
+class Test_Cachify_HDD extends WP_UnitTestCase {
+	/**
+	 * Test backend availability.
+	 */
+	public function test_is_available() {
+		update_option( 'permalink_structure', '' );
+		self::assertFalse( Cachify_HDD::is_available(), 'HDD backend should not be available without permalink structure' );
+
+		update_option( 'permalink_structure', '/%postname%/' );
+		self::assertTrue( Cachify_HDD::is_available(), 'HDD backend should be available without permalink structure' );
+	}
+
+	/**
+	 * Test backend availability.
+	 */
+	public function test_stringify_method() {
+		self::assertEquals( 'HDD', Cachify_HDD::stringify_method() );
+	}
+
+	/**
+	 * Test GZip availability.
+	 */
+	public function test_is_gzip_enabled() {
+		self::assertTrue( Cachify_HDD::is_gzip_enabled(), 'GZip should be enabled by default' );
+
+		$capture = null;
+		add_filter(
+			'cachify_create_gzip_files',
+			function ( $original ) use ( &$capture ) {
+				$capture = $original;
+
+				return false;
+			}
+		);
+		self::assertFalse( Cachify_HDD::is_gzip_enabled(), 'GZip should be disabled by filter' );
+		self::assertTrue( $capture, 'Filter was not applied' );
+	}
+
+	/**
+	 * Test the actual caching.
+	 */
+	public function test_caching() {
+		self::assertFalse( Cachify_HDD::get_item() );
+
+		self::go_to( '/testme/' );
+		Cachify_HDD::store_item(
+			'965b4abf2414e45036ab90c9d3f8dbc7', // Ignored.
+			'<html><head><title>Test Me</title></head><body><p>Test Content.</p></body></html>',
+			3600, // Ignored.
+			false
+		);
+		self::assertTrue( Cachify_HDD::get_item() );
+		self::assertTrue( is_file( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/testme/index.html' ) );
+		$cached = file_get_contents( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/testme/index.html' );
+		self::assertStringStartsWith(
+			'<html><head><title>Test Me</title></head><body><p>Test Content.</p></body></html>
+
+<!-- Cachify | https://cachify.pluginkollektiv.org
+Generated @ ',
+			$cached
+		);
+		self::assertStringEndsWith( ' -->', $cached );
+
+		// Another item.
+		self::go_to( '/test2/' );
+		Cachify_HDD::store_item(
+			'ef7e4a0540f6cde19e6eb658c69b0064', // Ignored.
+			'<html><head><title>Test 2</title></head><body><p>Test Content #2.</p></body></html>',
+			3600, // Ignored.
+			true
+		);
+		self::assertTrue( is_file( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/test2/index.html' ) );
+		$cached = file_get_contents( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/test2/index.html' );
+		self::assertStringStartsWith(
+			'<html><head><title>Test 2</title></head><body><p>Test Content #2.</p></body></html>
+
+<!-- Cachify | https://cachify.pluginkollektiv.org
+HDD Cache @ ',
+			$cached
+		);
+		self::assertStringEndsWith( ' -->', $cached );
+
+		// Delete the first item.
+		Cachify_HDD::delete_item( '965b4abf2414e45036ab90c9d3f8dbc7', 'http://example.org/testme/' );
+		self::assertFalse( is_file( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/testme/index.html' ), 'first item was not deleted' );
+		self::assertTrue( is_file( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/test2/index.html' ), 'second item should still be present' );
+
+		// Clear the cache.
+		Cachify_HDD::clear_cache();
+		self::assertFalse( is_dir( CACHIFY_CACHE_DIR . DIRECTORY_SEPARATOR . 'example.org/test2' ) );
+		self::assertFalse( Cachify_HDD::get_item() );
+	}
+}
diff --git a/tests/test-cachify-memcached.php b/tests/test-cachify-memcached.php
new file mode 100644
index 00000000..84cc0b58
--- /dev/null
+++ b/tests/test-cachify-memcached.php
@@ -0,0 +1,34 @@
+<?php
+/**
+ * Cachify Memcached tests.
+ *
+ * @package Cachify
+ */
+
+/**
+ * Class Test_Cachify_Memcached.
+ *
+ * Tests for the Cachify Memcached backend.
+ */
+class Test_Cachify_MEMCACHED extends WP_UnitTestCase {
+	/**
+	 * Test backend availability.
+	 */
+	public function test_is_available() {
+		/*
+		 * This test is a little tricky, because we need the PHP memcached extension loaded and only if we
+		 * are on a nginx webserver.
+		 * Let's just assume the equivalence between availability of the extension and the backend here.
+		 */
+		global $_SERVER;
+		$_SERVER['SERVER_SOFTWARE'] = 'nginx';
+		self::assertEquals( class_exists( 'Memcached' ), Cachify_MEMCACHED::is_available() );
+	}
+
+	/**
+	 * Test backend availability.
+	 */
+	public function test_stringify_method() {
+		self::assertEquals( 'Memcached', Cachify_MEMCACHED::stringify_method() );
+	}
+}
diff --git a/tests/test-cachify.php b/tests/test-cachify.php
new file mode 100644
index 00000000..f32225eb
--- /dev/null
+++ b/tests/test-cachify.php
@@ -0,0 +1,107 @@
+<?php
+/**
+ * Cachify tests.
+ *
+ * @package Cachify
+ */
+
+/**
+ * Class Test_Cachify.
+ *
+ * Tests for generic Cachify routines.
+ */
+class Test_Cachify extends WP_UnitTestCase {
+	/**
+	 * Test registration of flush hooks.
+	 */
+	public function test_register_flush_cache_hooks() {
+		$original_capture = null;
+
+		// Add a filter hook.
+		add_filter(
+			'cachify_flush_cache_hooks',
+			function ( $original ) use ( &$original_capture ) {
+				$original_capture = $original;
+
+				return array(
+					'test_1' => 10,
+					'test_2' => 20,
+				);
+			}
+		);
+
+		// Call flush registration.
+		Cachify::register_flush_cache_hooks();
+
+		// Verify that the filter has been called.
+		self::assertNotNull( $original_capture, 'Filter not called' );
+		self::assertEquals( 12, count( $original_capture ), 'Unexpected number of default hooks' );
+		self::assertEmpty(
+			array_filter(
+				$original_capture,
+				function( $v ) {
+					return 10 !== $v;
+				}
+			),
+			'All default filters should have priority 10'
+		);
+
+		// Verify that the action has been hooked with given priority.
+		self::assertEquals(
+			10,
+			has_action( 'test_1', array( Cachify::class, 'flush_total_cache' ) ),
+			'Flush action not hooked as expected'
+		);
+		self::assertEquals(
+			20,
+			has_action( 'test_2', array( Cachify::class, 'flush_total_cache' ) ),
+			'Flush action not hooked as expected'
+		);
+	}
+
+	/**
+	 * Test registration of scripts.
+	 */
+	public function test_register_scripts() {
+		Cachify::register_scripts();
+		self::assertTrue( wp_script_is( 'cachify-admin-bar-flush', 'registered' ) );
+		$script = wp_scripts()->registered['cachify-admin-bar-flush'];
+		self::assertStringEndsWith(
+			'/js/admin-bar-flush.min.js',
+			$script->src,
+			'unexpected script source'
+		);
+	}
+
+	/**
+	 * Test registration of styles.
+	 */
+	public function test_register_styles() {
+		Cachify::register_styles();
+		self::assertTrue( wp_style_is( 'cachify-dashboard', 'registered' ) );
+		self::assertTrue( wp_style_is( 'cachify-admin-bar-flush', 'registered' ) );
+
+		$style = wp_styles()->registered['cachify-dashboard'];
+		self::assertStringEndsWith(
+			'/css/dashboard.min.css',
+			$style->src,
+			'unexpected dashboard style source'
+		);
+
+		$style = wp_styles()->registered['cachify-admin-bar-flush'];
+		self::assertStringEndsWith(
+			'/css/admin-bar-flush.min.css',
+			$style->src,
+			'unexpected admin bar style source'
+		);
+	}
+
+	/**
+	 * Test single site plugin activation.
+	 */
+	public function test_on_activation() {
+		self::assertFalse( get_option( 'cachify' ), 'Cachify option should not be initialized initially' );
+		Cachify::on_activation();
+		self::assertEquals( array() , get_option( 'cachify' ), 'Cachify option not initialized' );
+	}
+}