diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 160a7f7a4d..b96ed477e7 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -50,6 +50,14 @@ if ( version_compare( $phpunit_version, '5.7', '<' ) || version_compare( $phpuni
 require_once __DIR__ . '/class-mockobject-autoload.php';
 spl_autoload_register( 'MockObject_Autoload::load', true, true );
 
+// Check that the PHPUnit Polyfills autoloader exists.
+$phpunit_polyfills_autoloader = __DIR__ . '/../../../vendor/yoast/phpunit-polyfills/phpunitpolyfills-autoload.php';
+if ( ! file_exists( $phpunit_polyfills_autoloader ) ) {
+	echo "Error: You need to run `composer update` before running the tests.\n";
+	echo "You can still use a PHPUnit phar to run them, but the dependencies do need to be installed.\n";
+	exit( 1 );
+}
+
 // If running core tests, check if all the required PHP extensions are loaded before running the test suite.
 if ( defined( 'WP_RUN_CORE_TESTS' ) && WP_RUN_CORE_TESTS ) {
 	$required_extensions = array(
@@ -195,13 +203,12 @@ if ( version_compare( tests_get_phpunit_version(), '6.0', '>=' ) ) {
 	require __DIR__ . '/phpunit6/compat.php';
 }
 
-// Load separate WP_UnitTestCase classes for PHPUnit 7.5+ and older versions.
-if ( version_compare( tests_get_phpunit_version(), '7.5', '>=' ) ) {
-	require __DIR__ . '/phpunit7/testcase.php';
-} else {
-	require __DIR__ . '/testcase.php';
-}
+// Load the PHPUnit Polyfills autoloader (check for existence of the file is done earlier in the script).
+require_once $phpunit_polyfills_autoloader;
+unset( $phpunit_polyfills_autoloader );
 
+require __DIR__ . '/abstract-testcase.php';
+require __DIR__ . '/testcase.php';
 require __DIR__ . '/testcase-rest-api.php';
 require __DIR__ . '/testcase-rest-controller.php';
 require __DIR__ . '/testcase-rest-post-type-controller.php';
diff --git a/tests/phpunit/includes/phpunit7/testcase.php b/tests/phpunit/includes/phpunit7/testcase.php
deleted file mode 100644
index 84ddbd5be1..0000000000
--- a/tests/phpunit/includes/phpunit7/testcase.php
+++ /dev/null
@@ -1,14 +0,0 @@
-<?php
-
-require_once dirname( __DIR__ ) . '/abstract-testcase.php';
-
-/**
- * Defines a basic fixture to run multiple tests.
- *
- * Resets the state of the WordPress installation before and after every test.
- *
- * Includes utility functions and assertions useful for testing WordPress.
- *
- * All WordPress unit tests should inherit from this class.
- */
-class WP_UnitTestCase extends WP_UnitTestCase_Base {}
diff --git a/tests/phpunit/includes/testcase.php b/tests/phpunit/includes/testcase.php
index 248d853132..1350f10e83 100644
--- a/tests/phpunit/includes/testcase.php
+++ b/tests/phpunit/includes/testcase.php
@@ -1,439 +1,48 @@
 <?php
 
-require_once __DIR__ . '/abstract-testcase.php';
+use Yoast\PHPUnitPolyfills\Helpers\AssertAttributeHelper;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertClosedResource;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertEqualsSpecializations;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertFileDirectory;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertFileEqualsSpecializations;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertionRenames;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertIsType;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertNumericType;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertObjectEquals;
+use Yoast\PHPUnitPolyfills\Polyfills\AssertStringContains;
+use Yoast\PHPUnitPolyfills\Polyfills\EqualToSpecializations;
+use Yoast\PHPUnitPolyfills\Polyfills\ExpectException;
+use Yoast\PHPUnitPolyfills\Polyfills\ExpectExceptionMessageMatches;
+use Yoast\PHPUnitPolyfills\Polyfills\ExpectExceptionObject;
+use Yoast\PHPUnitPolyfills\Polyfills\ExpectPHPException;
 
 /**
- * Defines a basic fixture to run multiple tests.
+ * Basic abstract test class with PHPUnit cross-version adapter layer.
  *
- * Resets the state of the WordPress installation before and after every test.
+ * This adapter layer polyfills all PHPUnit assertion and expectation
+ * methods which were added between PHPUnit 4.8 - 9.5 to allow tests
+ * to benefit from the full range of available PHPUnit methods.
  *
- * Includes utility functions and assertions useful for testing WordPress.
+ * See {@link https://github.com/Yoast/PHPUnit-Polyfills} for full
+ * documentation on the available polyfills.
  *
  * All WordPress unit tests should inherit from this class.
  */
 class WP_UnitTestCase extends WP_UnitTestCase_Base {
 
-	/**
-	 * Asserts that two variables are equal (with delta).
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.6.0
-	 *
-	 * @param mixed  $expected First value to compare.
-	 * @param mixed  $actual   Second value to compare.
-	 * @param float  $delta    Allowed numerical distance between two values to consider them equal.
-	 * @param string $message  Optional. Message to display when the assertion fails.
-	 *
-	 * @throws ExpectationFailedException
-	 * @throws \SebastianBergmann\RecursionContext\InvalidArgumentException
-	 */
-	public static function assertEqualsWithDelta( $expected, $actual, $delta, $message = '' ) {
-		static::assertEquals( $expected, $actual, $message, $delta );
-	}
-
-	/**
-	 * Asserts that a variable is of type array.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsArray( $actual, $message = '' ) {
-		static::assertInternalType( 'array', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type bool.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsBool( $actual, $message = '' ) {
-		static::assertInternalType( 'bool', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type float.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsFloat( $actual, $message = '' ) {
-		static::assertInternalType( 'float', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type int.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsInt( $actual, $message = '' ) {
-		static::assertInternalType( 'int', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type numeric.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNumeric( $actual, $message = '' ) {
-		static::assertInternalType( 'numeric', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type object.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsObject( $actual, $message = '' ) {
-		static::assertInternalType( 'object', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type resource.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsResource( $actual, $message = '' ) {
-		static::assertInternalType( 'resource', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type string.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsString( $actual, $message = '' ) {
-		static::assertInternalType( 'string', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type scalar.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsScalar( $actual, $message = '' ) {
-		static::assertInternalType( 'scalar', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type callable.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsCallable( $actual, $message = '' ) {
-		static::assertInternalType( 'callable', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is of type iterable.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * Support for `iterable` was only added to `Assert::assertNotInternalType()`
-	 * in PHPUnit 7.1.0, so this polyfill cannot use a direct fall-through
-	 * to that functionality until WordPress test suite requires PHPUnit 7.1.0
-	 * as the minimum version.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsIterable( $actual, $message = '' ) {
-		static::assertTrue( is_iterable( $actual ), $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type array.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotArray( $actual, $message = '' ) {
-		static::assertNotInternalType( 'array', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type bool.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotBool( $actual, $message = '' ) {
-		static::assertNotInternalType( 'bool', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type float.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotFloat( $actual, $message = '' ) {
-		static::assertNotInternalType( 'float', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type int.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotInt( $actual, $message = '' ) {
-		static::assertNotInternalType( 'int', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type numeric.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotNumeric( $actual, $message = '' ) {
-		static::assertNotInternalType( 'numeric', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type object.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotObject( $actual, $message = '' ) {
-		static::assertNotInternalType( 'object', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type resource.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotResource( $actual, $message = '' ) {
-		static::assertNotInternalType( 'resource', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type string.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotString( $actual, $message = '' ) {
-		static::assertNotInternalType( 'string', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type scalar.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotScalar( $actual, $message = '' ) {
-		static::assertNotInternalType( 'scalar', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type callable.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotCallable( $actual, $message = '' ) {
-		static::assertNotInternalType( 'callable', $actual, $message );
-	}
-
-	/**
-	 * Asserts that a variable is not of type iterable.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * Support for `iterable` was only added to `Assert::assertNotInternalType()`
-	 * in PHPUnit 7.1.0, so this polyfill cannot use a direct fall-through
-	 * to that functionality until WordPress test suite requires PHPUnit 7.1.0
-	 * as the minimum version.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param mixed  $actual  The value to check.
-	 * @param string $message Optional. Message to display when the assertion fails.
-	 */
-	public static function assertIsNotIterable( $actual, $message = '' ) {
-		static::assertFalse( is_iterable( $actual ), $message );
-	}
-
-	/**
-	 * Asserts that a string haystack contains a needle.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param string $needle   The string to search for.
-	 * @param string $haystack The string to treat as the haystack.
-	 * @param string $message  Optional. Message to display when the assertion fails.
-	 */
-	public static function assertStringContainsString( $needle, $haystack, $message = '' ) {
-		static::assertContains( $needle, $haystack, $message );
-	}
-
-	/**
-	 * Asserts that a string haystack contains a needle (case-insensitive).
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param string $needle   The string to search for.
-	 * @param string $haystack The string to treat as the haystack.
-	 * @param string $message  Optional. Message to display when the assertion fails.
-	 */
-	public static function assertStringContainsStringIgnoringCase( $needle, $haystack, $message = '' ) {
-		static::assertContains( $needle, $haystack, $message, true );
-	}
-
-	/**
-	 * Asserts that a string haystack does not contain a needle.
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param string $needle   The string to search for.
-	 * @param string $haystack The string to treat as the haystack.
-	 * @param string $message  Optional. Message to display when the assertion fails.
-	 */
-	public static function assertStringNotContainsString( $needle, $haystack, $message = '' ) {
-		static::assertNotContains( $needle, $haystack, $message );
-	}
-
-	/**
-	 * Asserts that a string haystack does not contain a needle (case-insensitive).
-	 *
-	 * This method has been backported from a more recent PHPUnit version,
-	 * as tests running on PHP 5.6 use PHPUnit 5.7.x.
-	 *
-	 * @since 5.9.0
-	 *
-	 * @param string $needle   The string to search for.
-	 * @param string $haystack The string to treat as the haystack.
-	 * @param string $message  Optional. Message to display when the assertion fails.
-	 */
-	public static function assertStringNotContainsStringIgnoringCase( $needle, $haystack, $message = '' ) {
-		static::assertNotContains( $needle, $haystack, $message, true );
-	}
+	use AssertAttributeHelper;
+	use AssertClosedResource;
+	use AssertEqualsSpecializations;
+	use AssertFileDirectory;
+	use AssertFileEqualsSpecializations;
+	use AssertionRenames;
+	use AssertIsType;
+	use AssertNumericType;
+	use AssertObjectEquals;
+	use AssertStringContains;
+	use EqualToSpecializations;
+	use ExpectException;
+	use ExpectExceptionMessageMatches;
+	use ExpectExceptionObject;
+	use ExpectPHPException;
 }