diff --git a/.env.example b/.env.example
index 76a4744165505..ac96332b0ae92 100644
--- a/.env.example
+++ b/.env.example
@@ -36,6 +36,9 @@ LOCAL_PHP_XDEBUG_MODE=develop,debug
 # Whether or not to enable Memcached.
 LOCAL_PHP_MEMCACHED=false
 
+# Whether or not to enable PCOV.
+LOCAL_PHP_PCOV=false
+
 ##
 # The database software to use.
 #
@@ -46,12 +49,12 @@ LOCAL_DB_TYPE=mysql
 ##
 # The database version to use.
 #
-# Defaults to 8.0 with the assumption that LOCAL_DB_TYPE is set to `mysql` above.
+# Defaults to 9.7 with the assumption that LOCAL_DB_TYPE is set to `mysql` above.
 #
 # When using `mysql`, see https://hub.docker.com/_/mysql for valid versions.
 # When using `mariadb`, see https://hub.docker.com/_/mariadb for valid versions.
 ##
-LOCAL_DB_VERSION=8.4
+LOCAL_DB_VERSION=9.7
 
 # Whether or not to enable multisite.
 LOCAL_MULTISITE=false
diff --git a/.github/workflows/install-testing.yml b/.github/workflows/install-testing.yml
index c55e2c6669f74..440c2ee842149 100644
--- a/.github/workflows/install-testing.yml
+++ b/.github/workflows/install-testing.yml
@@ -94,11 +94,12 @@ jobs:
           - db-version: '9.3'
           - db-version: '9.4'
           - db-version: '9.5'
+          - db-version: '9.6'
           # MySQL 9.0+ will not work on PHP 7.2 & 7.3. See https://core.trac.wordpress.org/ticket/61218.
           - php: '7.2'
-            db-version: '9.6'
+            db-version: '9.7'
           - php: '7.3'
-            db-version: '9.6'
+            db-version: '9.7'
 
     services:
       database:
diff --git a/.github/workflows/local-docker-environment.yml b/.github/workflows/local-docker-environment.yml
index d42bba623ec64..b786cc2419c03 100644
--- a/.github/workflows/local-docker-environment.yml
+++ b/.github/workflows/local-docker-environment.yml
@@ -108,6 +108,7 @@ jobs:
           - db-version: '9.3'
           - db-version: '9.4'
           - db-version: '9.5'
+          - db-version: '9.6'
           # No PHP 8.5 + Memcached support yet.
           - php: '8.5'
             memcached: true
diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index 74dfc220c04a6..f46b419656a44 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -76,7 +76,7 @@ jobs:
         os: [ ubuntu-24.04 ]
         php: [ '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5' ]
         db-type: [ 'mysql' ]
-        db-version: [ '5.7', '8.0', '8.4' ]
+        db-version: [ '5.7', '8.0', '8.4', '9.7' ]
         tests-domain: [ 'example.org' ]
         multisite: [ false, true ]
         memcached: [ false ]
@@ -209,15 +209,13 @@ jobs:
         os: [ ubuntu-24.04 ]
         php: [ '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5' ]
         db-type: [ 'mysql', 'mariadb' ]
-        db-version: [ '9.6', '12.1' ]
+        db-version: [ '12.1' ]
         multisite: [ false, true ]
         memcached: [ false ]
         db-innovation: [ true ]
 
         exclude:
           # Exclude version combinations that don't exist.
-          - db-type: 'mariadb'
-            db-version: '9.6'
           - db-type: 'mysql'
             db-version: '12.1'
     with:
@@ -283,14 +281,14 @@ jobs:
       fail-fast: false
       matrix:
         php: [ '7.4', '8.4' ]
-        db-version: [ '8.4', '11.8' ]
+        db-version: [ '9.7', '11.8' ]
         db-type: [ 'mysql', 'mariadb' ]
         multisite: [ false ]
 
         include:
           # Include one multisite job for each database type.
           - php: '8.4'
-            db-version: '8.4'
+            db-version: '9.7'
             db-type: 'mysql'
             multisite: true
           - php: '8.4'
@@ -299,13 +297,13 @@ jobs:
             multisite: true
           # Test with memcached.
           - php: '8.4'
-            db-version: '8.4'
+            db-version: '9.7'
             db-type: 'mysql'
             multisite: true
             memcached: true
           # Run specific test groups once.
           - php: '8.4'
-            db-version: '8.4'
+            db-version: '9.7'
             db-type: 'mysql'
             phpunit-test-groups: 'html-api-html5lib-tests'
 
@@ -314,7 +312,7 @@ jobs:
           - db-type: 'mysql'
             db-version: '11.8'
           - db-type: 'mariadb'
-            db-version: '8.4'
+            db-version: '9.7'
 
     with:
       php: ${{ matrix.php }}
diff --git a/.github/workflows/reusable-performance-test-v2.yml b/.github/workflows/reusable-performance-test-v2.yml
index d5617a66c17d5..c0279c37fe64b 100644
--- a/.github/workflows/reusable-performance-test-v2.yml
+++ b/.github/workflows/reusable-performance-test-v2.yml
@@ -78,6 +78,7 @@ jobs:
   # - Configure environment variables.
   # - Checkout repository.
   # - Set up Node.js.
+  # - Set up PHP.
   # - Log debug information.
   # - Install npm dependencies.
   # - Install Playwright browsers.
@@ -127,6 +128,14 @@ jobs:
           node-version-file: '.nvmrc'
           cache: npm
 
+      - name: Set up PHP
+        if: ${{ inputs.subject == 'base' }}
+        uses: shivammathur/setup-php@7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc # 2.37.1
+        with:
+          php-version: ${{ inputs.php-version }}
+          tools: wp-cli
+          coverage: none
+
       - name: Log debug information
         run: |
           npm --version
@@ -183,7 +192,7 @@ jobs:
         if: ${{ inputs.subject == 'base' }}
         run: |
           VERSION="${BASE_TAG%.0}"
-          npm run env:cli -- core download --version="$VERSION" --force --path="/var/www/${LOCAL_DIR}"
+          wp core download --version="$VERSION" --force --path="${LOCAL_DIR}"
 
       - name: Install object cache drop-in
         if: ${{ inputs.memcached }}
diff --git a/.github/workflows/reusable-phpunit-tests-v3.yml b/.github/workflows/reusable-phpunit-tests-v3.yml
index b75a6e37f91b4..e06f9f24be17e 100644
--- a/.github/workflows/reusable-phpunit-tests-v3.yml
+++ b/.github/workflows/reusable-phpunit-tests-v3.yml
@@ -82,8 +82,9 @@ on:
 
 env:
   LOCAL_PHP: ${{ inputs.php }}-fpm
-  LOCAL_PHP_XDEBUG: ${{ inputs.coverage-report || false }}
-  LOCAL_PHP_XDEBUG_MODE: ${{ inputs.coverage-report && 'coverage' || 'develop,debug' }}
+  LOCAL_PHP_XDEBUG: false
+  LOCAL_PHP_XDEBUG_MODE: 'develop,debug'
+  LOCAL_PHP_PCOV: ${{ inputs.coverage-report }}
   LOCAL_DB_TYPE: ${{ inputs.db-type }}
   LOCAL_DB_VERSION: ${{ inputs.db-version }}
   LOCAL_PHP_MEMCACHED: ${{ inputs.memcached }}
@@ -113,7 +114,6 @@ jobs:
   # - Install WordPress within the Docker container.
   # - Run the PHPUnit tests.
   # - Upload the code coverage report to Codecov.io.
-  # - Upload the HTML code coverage report as an artifact.
   # - Ensures version-controlled files are not modified or deleted.
   # - Checks out the WordPress Test reporter repository.
   # - Submit the test results to the WordPress.org host test results.
@@ -204,12 +204,12 @@ jobs:
       - name: Run PHPUnit tests${{ inputs.phpunit-test-groups && format( ' ({0} groups)', inputs.phpunit-test-groups ) || '' }}${{ inputs.coverage-report && ' with coverage report' || '' }}
         continue-on-error: ${{ inputs.allow-errors }}
         run: |
-          node ./tools/local-env/scripts/docker.js run \
+          node ./tools/local-env/scripts/docker.js ${{ inputs.coverage-report && 'exec' || 'run' }} \
             php ./vendor/bin/phpunit \
             --verbose \
             -c "${PHPUNIT_CONFIG}" \
             ${{ inputs.phpunit-test-groups && '--group "${TEST_GROUPS}"' || '' }} \
-            ${{ inputs.coverage-report && '--coverage-clover "wp-code-coverage-${MULTISITE_FLAG}-${GITHUB_SHA}.xml" --coverage-html "wp-code-coverage-${MULTISITE_FLAG}-${GITHUB_SHA}"' || '' }}
+            ${{ inputs.coverage-report && '--coverage-clover "wp-code-coverage-${MULTISITE_FLAG}-${GITHUB_SHA}.xml"' || '' }}
         env:
           TEST_GROUPS: ${{ inputs.phpunit-test-groups }}
           MULTISITE_FLAG: ${{ inputs.multisite && 'multisite' || 'single' }}
@@ -244,14 +244,6 @@ jobs:
           flags: ${{ inputs.multisite && 'multisite' || 'single' }},php
           fail_ci_if_error: true
 
-      - name: Upload HTML coverage report as artifact
-        if: ${{ inputs.coverage-report }}
-        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
-        with:
-          name: wp-code-coverage${{ inputs.multisite && '-multisite' || '-single' }}-${{ github.sha }}
-          path: wp-code-coverage${{ inputs.multisite && '-multisite' || '-single' }}-${{ github.sha }}
-          overwrite: true
-
       - name: Ensure version-controlled files are not modified or deleted
         run: git diff --exit-code
 
diff --git a/.github/workflows/test-coverage.yml b/.github/workflows/test-coverage.yml
index f2b0afce3256f..e1c3dd3be2f59 100644
--- a/.github/workflows/test-coverage.yml
+++ b/.github/workflows/test-coverage.yml
@@ -38,8 +38,6 @@ concurrency:
 permissions: {}
 
 env:
-  LOCAL_PHP_XDEBUG: true
-  LOCAL_PHP_XDEBUG_MODE: 'coverage'
   LOCAL_PHP_MEMCACHED: false
   PUPPETEER_SKIP_DOWNLOAD: true
 
diff --git a/.github/workflows/upgrade-testing.yml b/.github/workflows/upgrade-testing.yml
index ce80cfb6f0989..8a74e5ae614c5 100644
--- a/.github/workflows/upgrade-testing.yml
+++ b/.github/workflows/upgrade-testing.yml
@@ -70,7 +70,7 @@ jobs:
         os: [ 'ubuntu-24.04' ]
         php: [ '7.4', '8.0', '8.1', '8.2', '8.3', '8.4', '8.5' ]
         db-type: [ 'mysql' ]
-        db-version: [ '5.7', '8.0', '8.4', '9.6' ]
+        db-version: [ '5.7', '8.0', '8.4', '9.7' ]
         wp: [ '6.9', '7.0' ]
         multisite: [ false, true ]
     with:
@@ -179,7 +179,7 @@ jobs:
         os: [ 'ubuntu-24.04' ]
         php: [ '7.4' ]
         db-type: [ 'mysql' ]
-        db-version: [ '5.7', '8.0', '8.4', '9.6' ]
+        db-version: [ '5.7', '8.0', '8.4', '9.7' ]
         wp: [ '4.7' ]
         multisite: [ false, true ]
     with:
diff --git a/.version-support-mysql.json b/.version-support-mysql.json
index 6e81f2eff0f09..7ab8aa38df392 100644
--- a/.version-support-mysql.json
+++ b/.version-support-mysql.json
@@ -1,12 +1,6 @@
 {
 	"7-1": [
-		"9.6",
-		"9.5",
-		"9.4",
-		"9.3",
-		"9.2",
-		"9.1",
-		"9.0",
+		"9.7",
 		"8.4",
 		"8.0",
 		"5.7",
diff --git a/docker-compose.yml b/docker-compose.yml
index cc2ed8d94975e..0af433a972b13 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -32,7 +32,7 @@ services:
   # The PHP container.
   ##
   php:
-    image: wordpressdevelop/php:${LOCAL_PHP-latest}
+    image: ghcr.io/wordpress/wpdev-docker-images/php:${LOCAL_PHP-latest}-210
 
     networks:
       - wpdevnet
@@ -40,6 +40,7 @@ services:
     environment:
       LOCAL_PHP_XDEBUG: ${LOCAL_PHP_XDEBUG-false}
       XDEBUG_MODE: ${LOCAL_PHP_XDEBUG_MODE-develop,debug}
+      LOCAL_PHP_PCOV: ${LOCAL_PHP_PCOV-false}
       LOCAL_PHP_MEMCACHED: ${LOCAL_PHP_MEMCACHED-false}
       PHP_FPM_UID: ${PHP_FPM_UID-1000}
       PHP_FPM_GID: ${PHP_FPM_GID-1000}
@@ -92,13 +93,14 @@ services:
   # The WP CLI container.
   ##
   cli:
-    image: wordpressdevelop/cli:${LOCAL_PHP-latest}
+    image: ghcr.io/wordpress/wpdev-docker-images/cli:${LOCAL_PHP-latest}-210
 
     networks:
       - wpdevnet
 
     environment:
       LOCAL_PHP_XDEBUG: ${LOCAL_PHP_XDEBUG-false}
+      LOCAL_PHP_PCOV: ${LOCAL_PHP_PCOV-false}
       LOCAL_PHP_MEMCACHED: ${LOCAL_PHP_MEMCACHED-false}
       PHP_FPM_UID: ${PHP_FPM_UID-1000}
       PHP_FPM_GID: ${PHP_FPM_GID-1000}
diff --git a/src/wp-includes/class-wp-hook.php b/src/wp-includes/class-wp-hook.php
index cd6860c0f81f2..2ce6ee0a17648 100644
--- a/src/wp-includes/class-wp-hook.php
+++ b/src/wp-includes/class-wp-hook.php
@@ -85,6 +85,9 @@ public function add_filter( $hook_name, $callback, $priority, $accepted_args ) {
 		}
 
 		$idx = _wp_filter_build_unique_id( $hook_name, $callback, $priority );
+		if ( null === $idx ) {
+			return;
+		}
 
 		$priority_existed = isset( $this->callbacks[ $priority ] );
 
diff --git a/src/wp-includes/class-wp-widget-factory.php b/src/wp-includes/class-wp-widget-factory.php
index ed719b9d4d7a1..b6233eabba002 100644
--- a/src/wp-includes/class-wp-widget-factory.php
+++ b/src/wp-includes/class-wp-widget-factory.php
@@ -57,7 +57,7 @@ public function WP_Widget_Factory() {
 	 */
 	public function register( $widget ) {
 		if ( $widget instanceof WP_Widget ) {
-			$this->widgets[ spl_object_hash( $widget ) ] = $widget;
+			$this->widgets[ spl_object_id( $widget ) ] = $widget;
 		} else {
 			$this->widgets[ $widget ] = new $widget();
 		}
@@ -74,7 +74,7 @@ public function register( $widget ) {
 	 */
 	public function unregister( $widget ) {
 		if ( $widget instanceof WP_Widget ) {
-			unset( $this->widgets[ spl_object_hash( $widget ) ] );
+			unset( $this->widgets[ spl_object_id( $widget ) ] );
 		} else {
 			unset( $this->widgets[ $widget ] );
 		}
diff --git a/src/wp-includes/plugin.php b/src/wp-includes/plugin.php
index e980af5a09f66..55459c0dd96c8 100644
--- a/src/wp-includes/plugin.php
+++ b/src/wp-includes/plugin.php
@@ -986,33 +986,36 @@ function _wp_call_all_hook( $args ) {
  * @since 2.2.3
  * @since 5.3.0 Removed workarounds for spl_object_hash().
  *              `$hook_name` and `$priority` are no longer used,
- *              and the function always returns a string.
+ *              and no longer returns false, but can still return void for invalid callbacks.
+ * @since 6.9.0 Returns explicit null if an invalid callback is supplied.
+ * @since 7.1.0 Uses spl_object_id() instead of spl_object_hash() for performance.
  *
  * @access private
  *
- * @param string                $hook_name Unused. The name of the filter to build ID for.
- * @param callable|string|array $callback  The callback to generate ID for. The callback may
- *                                         or may not exist.
- * @param int                   $priority  Unused. The order in which the functions
- *                                         associated with a particular action are executed.
- * @return string|null Unique function ID for usage as array key.
- *                     Null if a valid `$callback` is not passed.
+ * @param string   $hook_name Unused. The name of the filter to build ID for.
+ * @param callable $callback  The callback to generate ID for. The callback may
+ *                            or may not exist.
+ * @param int      $priority  Unused. The order in which the functions
+ *                            associated with a particular action are executed.
+ * @return string|null Unique function ID for usage as array key, or null if it couldn't be determined.
  */
-function _wp_filter_build_unique_id( $hook_name, $callback, $priority ) {
+function _wp_filter_build_unique_id( $hook_name, $callback, $priority ): ?string {
 	if ( is_string( $callback ) ) {
 		return $callback;
 	}
 
 	if ( is_object( $callback ) ) {
-		// Closures are currently implemented as objects.
-		$callback = array( $callback, '' );
-	} else {
-		$callback = (array) $callback;
+		return (string) spl_object_id( (object) $callback );
+	}
+
+	$callback = (array) $callback;
+	if ( ! isset( $callback[1] ) || ! is_string( $callback[1] ) ) {
+		return null;
 	}
 
 	if ( is_object( $callback[0] ) ) {
 		// Object class calling.
-		return spl_object_hash( $callback[0] ) . $callback[1];
+		return ( (string) spl_object_id( $callback[0] ) ) . $callback[1];
 	} elseif ( is_string( $callback[0] ) ) {
 		// Static calling.
 		return $callback[0] . '::' . $callback[1];
diff --git a/tests/phpunit/tests/abilities-api/wpAbility.php b/tests/phpunit/tests/abilities-api/wpAbility.php
index 926a0e56a5280..73c4f9db43ffd 100644
--- a/tests/phpunit/tests/abilities-api/wpAbility.php
+++ b/tests/phpunit/tests/abilities-api/wpAbility.php
@@ -568,18 +568,19 @@ public function test_before_execute_ability_action() {
 			)
 		);
 
-		$callback = static function ( $ability_name, $input ) use ( &$action_ability_name, &$action_input ) {
-			$action_ability_name = $ability_name;
-			$action_input        = $input;
-		};
-
-		add_action( 'wp_before_execute_ability', $callback, 10, 2 );
+		add_action(
+			'wp_before_execute_ability',
+			static function ( $ability_name, $input ) use ( &$action_ability_name, &$action_input ) {
+				$action_ability_name = $ability_name;
+				$action_input        = $input;
+			},
+			10,
+			2
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute( 5 );
 
-		remove_action( 'wp_before_execute_ability', $callback );
-
 		$this->assertSame( self::$test_ability_name, $action_ability_name, 'Action should receive correct ability name' );
 		$this->assertSame( 5, $action_input, 'Action should receive correct input' );
 		$this->assertSame( 10, $result, 'Ability should execute correctly' );
@@ -603,18 +604,19 @@ public function test_before_execute_ability_action_no_input() {
 			)
 		);
 
-		$callback = static function ( $ability_name, $input ) use ( &$action_ability_name, &$action_input ) {
-			$action_ability_name = $ability_name;
-			$action_input        = $input;
-		};
-
-		add_action( 'wp_before_execute_ability', $callback, 10, 2 );
+		add_action(
+			'wp_before_execute_ability',
+			static function ( $ability_name, $input ) use ( &$action_ability_name, &$action_input ) {
+				$action_ability_name = $ability_name;
+				$action_input        = $input;
+			},
+			10,
+			2
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute();
 
-		remove_action( 'wp_before_execute_ability', $callback );
-
 		$this->assertSame( self::$test_ability_name, $action_ability_name, 'Action should receive correct ability name' );
 		$this->assertNull( $action_input, 'Action should receive null input when no input provided' );
 		$this->assertSame( 42, $result, 'Ability should execute correctly' );
@@ -644,19 +646,20 @@ public function test_after_execute_ability_action() {
 			)
 		);
 
-		$callback = static function ( $ability_name, $input, $result ) use ( &$action_ability_name, &$action_input, &$action_result ) {
-			$action_ability_name = $ability_name;
-			$action_input        = $input;
-			$action_result       = $result;
-		};
-
-		add_action( 'wp_after_execute_ability', $callback, 10, 3 );
+		add_action(
+			'wp_after_execute_ability',
+			static function ( $ability_name, $input, $result ) use ( &$action_ability_name, &$action_input, &$action_result ) {
+				$action_ability_name = $ability_name;
+				$action_input        = $input;
+				$action_result       = $result;
+			},
+			10,
+			3
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute( 7 );
 
-		remove_action( 'wp_after_execute_ability', $callback );
-
 		$this->assertSame( self::$test_ability_name, $action_ability_name, 'Action should receive correct ability name' );
 		$this->assertSame( 7, $action_input, 'Action should receive correct input' );
 		$this->assertSame( 21, $action_result, 'Action should receive correct result' );
@@ -683,19 +686,20 @@ public function test_after_execute_ability_action_no_input() {
 			)
 		);
 
-		$callback = static function ( $ability_name, $input, $result ) use ( &$action_ability_name, &$action_input, &$action_result ) {
-			$action_ability_name = $ability_name;
-			$action_input        = $input;
-			$action_result       = $result;
-		};
-
-		add_action( 'wp_after_execute_ability', $callback, 10, 3 );
+		add_action(
+			'wp_after_execute_ability',
+			static function ( $ability_name, $input, $result ) use ( &$action_ability_name, &$action_input, &$action_result ) {
+				$action_ability_name = $ability_name;
+				$action_input        = $input;
+				$action_result       = $result;
+			},
+			10,
+			3
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute();
 
-		remove_action( 'wp_after_execute_ability', $callback );
-
 		$this->assertSame( self::$test_ability_name, $action_ability_name, 'Action should receive correct ability name' );
 		$this->assertNull( $action_input, 'Action should receive null input when no input provided' );
 		$this->assertSame( 'test-result', $action_result, 'Action should receive correct result' );
@@ -720,23 +724,23 @@ public function test_actions_not_fired_on_permission_failure() {
 			)
 		);
 
-		$before_callback = static function () use ( &$before_action_fired ) {
-			$before_action_fired = true;
-		};
-
-		$after_callback = static function () use ( &$after_action_fired ) {
-			$after_action_fired = true;
-		};
+		add_action(
+			'wp_before_execute_ability',
+			static function () use ( &$before_action_fired ) {
+				$before_action_fired = true;
+			}
+		);
 
-		add_action( 'wp_before_execute_ability', $before_callback );
-		add_action( 'wp_after_execute_ability', $after_callback );
+		add_action(
+			'wp_after_execute_ability',
+			static function () use ( &$after_action_fired ) {
+				$after_action_fired = true;
+			}
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute();
 
-		remove_action( 'wp_before_execute_ability', $before_callback );
-		remove_action( 'wp_after_execute_ability', $after_callback );
-
 		$this->assertFalse( $before_action_fired, 'before_execute_ability action should not be fired on permission failure' );
 		$this->assertFalse( $after_action_fired, 'after_execute_ability action should not be fired on permission failure' );
 		$this->assertInstanceOf( WP_Error::class, $result, 'Should return WP_Error on permission failure' );
@@ -760,23 +764,23 @@ public function test_after_action_not_fired_on_execution_error() {
 			)
 		);
 
-		$before_callback = static function () use ( &$before_action_fired ) {
-			$before_action_fired = true;
-		};
-
-		$after_callback = static function () use ( &$after_action_fired ) {
-			$after_action_fired = true;
-		};
+		add_action(
+			'wp_before_execute_ability',
+			static function () use ( &$before_action_fired ) {
+				$before_action_fired = true;
+			}
+		);
 
-		add_action( 'wp_before_execute_ability', $before_callback );
-		add_action( 'wp_after_execute_ability', $after_callback );
+		add_action(
+			'wp_after_execute_ability',
+			static function () use ( &$after_action_fired ) {
+				$after_action_fired = true;
+			}
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute();
 
-		remove_action( 'wp_before_execute_ability', $before_callback );
-		remove_action( 'wp_after_execute_ability', $after_callback );
-
 		$this->assertTrue( $before_action_fired, 'before_execute_ability action should be fired even if execution fails' );
 		$this->assertFalse( $after_action_fired, 'after_execute_ability action should not be fired when execution returns WP_Error' );
 		$this->assertInstanceOf( WP_Error::class, $result, 'Should return WP_Error from execution callback' );
@@ -805,23 +809,23 @@ public function test_after_action_not_fired_on_output_validation_error() {
 			)
 		);
 
-		$before_callback = static function () use ( &$before_action_fired ) {
-			$before_action_fired = true;
-		};
-
-		$after_callback = static function () use ( &$after_action_fired ) {
-			$after_action_fired = true;
-		};
+		add_action(
+			'wp_before_execute_ability',
+			static function () use ( &$before_action_fired ) {
+				$before_action_fired = true;
+			}
+		);
 
-		add_action( 'wp_before_execute_ability', $before_callback );
-		add_action( 'wp_after_execute_ability', $after_callback );
+		add_action(
+			'wp_after_execute_ability',
+			static function () use ( &$after_action_fired ) {
+				$after_action_fired = true;
+			}
+		);
 
 		$ability = new WP_Ability( self::$test_ability_name, $args );
 		$result  = $ability->execute();
 
-		remove_action( 'wp_before_execute_ability', $before_callback );
-		remove_action( 'wp_after_execute_ability', $after_callback );
-
 		$this->assertTrue( $before_action_fired, 'before_execute_ability action should be fired even if output validation fails' );
 		$this->assertFalse( $after_action_fired, 'after_execute_ability action should not be fired when output validation fails' );
 		$this->assertInstanceOf( WP_Error::class, $result, 'Should return WP_Error for output validation failure' );
diff --git a/tests/phpunit/tests/abilities-api/wpRegisterAbility.php b/tests/phpunit/tests/abilities-api/wpRegisterAbility.php
index 61bf8f59dba53..133343635cf59 100644
--- a/tests/phpunit/tests/abilities-api/wpRegisterAbility.php
+++ b/tests/phpunit/tests/abilities-api/wpRegisterAbility.php
@@ -521,13 +521,15 @@ public function test_get_ability_no_init_action(): void {
 	public function test_get_existing_ability_using_callback() {
 		$this->simulate_doing_wp_abilities_init_action();
 
-		$name     = self::$test_ability_name;
-		$args     = self::$test_ability_args;
-		$callback = static function ( $instance ) use ( $name, $args ) {
-			wp_register_ability( $name, $args );
-		};
+		$name = self::$test_ability_name;
+		$args = self::$test_ability_args;
 
-		add_action( 'wp_abilities_api_init', $callback );
+		add_action(
+			'wp_abilities_api_init',
+			static function ( $instance ) use ( $name, $args ) {
+				wp_register_ability( $name, $args );
+			}
+		);
 
 		// Reset the Registry, to ensure it's empty before the test.
 		$registry_reflection = new ReflectionClass( WP_Abilities_Registry::class );
@@ -539,8 +541,6 @@ public function test_get_existing_ability_using_callback() {
 
 		$result = wp_get_ability( $name );
 
-		remove_action( 'wp_abilities_api_init', $callback );
-
 		$this->assertEquals(
 			new WP_Ability( $name, $args ),
 			$result,
diff --git a/tests/phpunit/tests/abilities-api/wpRegisterAbilityCategory.php b/tests/phpunit/tests/abilities-api/wpRegisterAbilityCategory.php
index 5a59b1050bf25..9b83928e19d5c 100644
--- a/tests/phpunit/tests/abilities-api/wpRegisterAbilityCategory.php
+++ b/tests/phpunit/tests/abilities-api/wpRegisterAbilityCategory.php
@@ -293,13 +293,15 @@ public function test_get_nonexistent_category(): void {
 	 * @ticket 64098
 	 */
 	public function test_get_existing_category_using_callback(): void {
-		$name     = self::$test_ability_category_name;
-		$args     = self::$test_ability_category_args;
-		$callback = static function ( $instance ) use ( $name, $args ) {
-			wp_register_ability_category( $name, $args );
-		};
+		$name = self::$test_ability_category_name;
+		$args = self::$test_ability_category_args;
 
-		add_action( 'wp_abilities_api_categories_init', $callback );
+		add_action(
+			'wp_abilities_api_categories_init',
+			static function ( $instance ) use ( $name, $args ) {
+				wp_register_ability_category( $name, $args );
+			}
+		);
 
 		// Reset the Registry, to ensure it's empty before the test.
 		$registry_reflection = new ReflectionClass( WP_Ability_Categories_Registry::class );
@@ -311,8 +313,6 @@ public function test_get_existing_category_using_callback(): void {
 
 		$result = wp_get_ability_category( $name );
 
-		remove_action( 'wp_abilities_api_categories_init', $callback );
-
 		$this->assertInstanceOf( WP_Ability_Category::class, $result );
 		$this->assertSame( self::$test_ability_category_name, $result->get_slug() );
 	}
diff --git a/tests/phpunit/tests/hooks/buildUniqueId.php b/tests/phpunit/tests/hooks/buildUniqueId.php
new file mode 100644
index 0000000000000..387a24bbc2fd1
--- /dev/null
+++ b/tests/phpunit/tests/hooks/buildUniqueId.php
@@ -0,0 +1,62 @@
+<?php
+
+/**
+ * Tests for _wp_filter_build_unique_id().
+ *
+ * @group hooks
+ * @covers ::_wp_filter_build_unique_id
+ */
+class Tests_Hooks_BuildUniqueId extends WP_UnitTestCase {
+
+	public function test_string_callback_returns_string(): void {
+		$result = _wp_filter_build_unique_id( '', '__return_null', 10 );
+		$this->assertIsString( $result );
+		$this->assertSame( '__return_null', $result );
+	}
+
+	public function test_closure_returns_string(): void {
+		$cb     = function (): void {};
+		$result = _wp_filter_build_unique_id( '', $cb, 10 );
+		$this->assertIsString( $result );
+	}
+
+	public function test_object_callback_returns_string(): void {
+		$a      = new MockAction();
+		$result = _wp_filter_build_unique_id( '', array( $a, 'action' ), 10 );
+		$this->assertIsString( $result );
+	}
+
+	public function test_static_callback_returns_string(): void {
+		$result = _wp_filter_build_unique_id( '', array( 'MockAction', 'action' ), 10 );
+		$this->assertIsString( $result );
+	}
+
+	public function test_two_different_objects_produce_different_ids(): void {
+		$a = new MockAction();
+		$b = new MockAction();
+		$this->assertNotSame(
+			_wp_filter_build_unique_id( '', array( $a, 'action' ), 10 ),
+			_wp_filter_build_unique_id( '', array( $b, 'action' ), 10 )
+		);
+	}
+
+	public function test_same_object_produces_same_id(): void {
+		$a = new MockAction();
+		$this->assertSame(
+			_wp_filter_build_unique_id( '', array( $a, 'action' ), 10 ),
+			_wp_filter_build_unique_id( '', array( $a, 'action' ), 10 )
+		);
+	}
+
+	public function test_malformed_array_missing_method_returns_null(): void {
+		$a      = new MockAction();
+		$result = _wp_filter_build_unique_id( '', array( $a ), 10 );
+		$this->assertNull( $result );
+	}
+
+	public function test_malformed_array_non_string_method_returns_null(): void {
+		$a      = new MockAction();
+		$result = _wp_filter_build_unique_id( '', array( $a, 123 ), 10 );
+		$this->assertNull( $result );
+	}
+}
diff --git a/tests/phpunit/tests/rest-api/wpRestAbilitiesV1CategoriesController.php b/tests/phpunit/tests/rest-api/wpRestAbilitiesV1CategoriesController.php
index 43525263ac5ba..f52fa32543d47 100644
--- a/tests/phpunit/tests/rest-api/wpRestAbilitiesV1CategoriesController.php
+++ b/tests/phpunit/tests/rest-api/wpRestAbilitiesV1CategoriesController.php
@@ -206,7 +206,6 @@ public function test_get_item_with_selected_fields(): void {
 		$response = $this->server->dispatch( $request );
 		add_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10, 3 );
 		$response = apply_filters( 'rest_post_dispatch', $response, $this->server, $request );
-		remove_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10 );
 
 		$this->assertEquals( 200, $response->get_status() );
 
diff --git a/tests/phpunit/tests/rest-api/wpRestAbilitiesV1ListController.php b/tests/phpunit/tests/rest-api/wpRestAbilitiesV1ListController.php
index d73a2c64177fc..1465319f234de 100644
--- a/tests/phpunit/tests/rest-api/wpRestAbilitiesV1ListController.php
+++ b/tests/phpunit/tests/rest-api/wpRestAbilitiesV1ListController.php
@@ -328,7 +328,6 @@ public function test_get_item_with_selected_fields(): void {
 		$response = $this->server->dispatch( $request );
 		add_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10, 3 );
 		$response = apply_filters( 'rest_post_dispatch', $response, $this->server, $request );
-		remove_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10 );
 
 		$this->assertEquals( 200, $response->get_status() );
 
@@ -349,7 +348,6 @@ public function test_get_item_with_embed_context(): void {
 		$response = $this->server->dispatch( $request );
 		add_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10, 3 );
 		$response = apply_filters( 'rest_post_dispatch', $response, $this->server, $request );
-		remove_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10 );
 
 		$this->assertEquals( 200, $response->get_status() );
 
diff --git a/tools/local-env/php-config.ini b/tools/local-env/php-config.ini
index 6f98802e5c68a..30ee177289f1d 100644
--- a/tools/local-env/php-config.ini
+++ b/tools/local-env/php-config.ini
@@ -5,3 +5,9 @@ post_max_size = 1G
 xdebug.start_with_request=trigger
 xdebug.discover_client_host=true
 xdebug.client_host=host.docker.internal
+
+# Pre-sizes the internal file tracking array for the thousands of files under
+# `src/` or `build/`, avoiding reallocation churn during test warmup.
+#
+# The default of 64 is far too low here.
+pcov.initial.files=10000
diff --git a/tools/local-env/scripts/docker.js b/tools/local-env/scripts/docker.js
index c7b11f0058424..bfe2e36136739 100644
--- a/tools/local-env/scripts/docker.js
+++ b/tools/local-env/scripts/docker.js
@@ -20,6 +20,25 @@ if ( [ 'exec', 'run' ].includes( dockerCommand[0] ) && ! process.stdin.isTTY ) {
 	dockerCommand.splice( 1, 0, '--no-TTY' );
 }
 
+/*
+ * `pcov.directory` restricts instrumentation to `src/`, so PCOV does not record hits for `vendor/`, `tests/`, or
+ * WordPress test fixtures that PHPUnit would discard at report time anyway.
+ */
+if ( process.env.LOCAL_PHP_PCOV === 'true' && dockerCommand.includes( '--coverage-clover' ) ) {
+	const phpunitIdx = dockerCommand.findIndex( ( arg ) => typeof arg === 'string' && arg.endsWith( 'phpunit' ) );
+	if ( phpunitIdx !== -1 ) {
+		const localDir = process.env.LOCAL_DIR || 'src';
+		dockerCommand.splice(
+			phpunitIdx,
+			1,
+			'php',
+			'-d',
+			`pcov.directory=/var/www/${ localDir }`,
+			dockerCommand[ phpunitIdx ]
+		);
+	}
+}
+
 // Add a --defaults flag to any db command WP-CLI command. See https://core.trac.wordpress.org/ticket/63876.
 if ( dockerCommand.includes( 'cli' ) && dockerCommand.includes( 'db' ) && ! dockerCommand.includes( '--defaults' ) ) {
 	dockerCommand.push( '--defaults' );