diff --git a/README.md b/README.md
index 6d2a390..d9b173a 100644
--- a/README.md
+++ b/README.md
@@ -579,7 +579,7 @@ class MyTest extends XTestCase {
 >
 > If you need the TestListener polyfill, it is recommended to stay on the PHPUnit Polyfills 1.x series for the time being and to watch and upvote the [related ticket][polyfill-ticket].
 >
-> The below documentation is for the PHPUnit 6.x-9.x TestListener polyfill implementation.
+> The below documentation is for the PHPUnit 5.x-9.x TestListener polyfill implementation.
 
 [polyfill-ticket]: https://github.com/Yoast/PHPUnit-Polyfills/issues/128
 
@@ -598,21 +598,24 @@ This `TestListenerDefaultImplementation` trait overcomes the signature mismatche
 
 Similar to the `TestCase` implementation, snake_case methods without type declarations are used to get round the signature mismatches. The snake_case methods will automatically be called.
 
-| PHPUnit native method name | Replacement                             |
-| -------------------------- | --------------------------------------- |
-| `addError()`               | `add_error($test, $e, $time)`           |
-| `addWarning()`             | `add_warning($test, $e, $time)`         |
-| `addFailure()`             | `add_failure($test, $e, $time)`         |
-| `addIncompleteTest()`      | `add_incomplete_test($test, $e, $time)` |
-| `addRiskyTest()`           | `add_risky_test($test, $e, $time)`      |
-| `addSkippedTest()`         | `add_skipped_test($test, $e, $time)`    |
-| `startTestSuite()`         | `start_test_suite($suite)`              |
-| `endTestSuite()`           | `end_test_suite($suite)`                |
-| `startTest()`              | `start_test($test)`                     |
-| `endTest()`                | `end_test($test, $time)`                |
+| PHPUnit native method name | Replacement                             | Notes                                     |
+| -------------------------- | --------------------------------------- | ----------------------------------------- |
+| `addError()`               | `add_error($test, $e, $time)`           |                                           |
+| `addWarning()`             | `add_warning($test, $e, $time)`         | Introduced in PHPUnit 6.                  |
+| `addFailure()`             | `add_failure($test, $e, $time)`         |                                           |
+| `addIncompleteTest()`      | `add_incomplete_test($test, $e, $time)` |                                           |
+| `addRiskyTest()`           | `add_risky_test($test, $e, $time)`      | Support appears to be flaky on PHPUnit 5. |
+| `addSkippedTest()`         | `add_skipped_test($test, $e, $time)`    |                                           |
+| `startTestSuite()`         | `start_test_suite($suite)`              |                                           |
+| `endTestSuite()`           | `end_test_suite($suite)`                |                                           |
+| `startTest()`              | `start_test($test)`                     |                                           |
+| `endTest()`                | `end_test($test, $time)`                |                                           |
 
 Implementations of the `TestListener` interface may be using any of the following patterns:
 ```php
+// PHPUnit < 6.
+class MyTestListener extends \PHPUnit_Framework_BaseTestListener {}
+
 // PHPUnit 6.
 class MyTestListener extends \PHPUnit\Framework\BaseTestListener {}
 
diff --git a/phpunitpolyfills-autoload.php b/phpunitpolyfills-autoload.php
index 6ddf1b8..273a5d1 100644
--- a/phpunitpolyfills-autoload.php
+++ b/phpunitpolyfills-autoload.php
@@ -337,8 +337,29 @@ public static function loadTestCase() {
 		 * @return void
 		 */
 		public static function loadTestListenerDefaultImplementation() {
+			if ( \version_compare( self::getPHPUnitVersion(), '6.0.0', '<' ) ) {
+				/*
+				 * Alias one particular PHPUnit 5.x class to its PHPUnit >= 6 name.
+				 *
+				 * All other classes needed are part of the forward-compatibility layer.
+				 *
+				 * {@internal The `class_exists` wrappers are needed to play nice with
+				 * PHPUnit bootstrap files of test suites implementing this library
+				 * which may be creating cross-version compatibility in a similar manner.}}
+				 */
+				if ( \class_exists( 'PHPUnit_Framework_Warning' ) === true
+					&& \class_exists( 'PHPUnit\Framework\Warning' ) === false
+				) {
+					\class_alias( 'PHPUnit_Framework_Warning', 'PHPUnit\Framework\Warning' );
+				}
+
+				// PHPUnit < 6.0.0.
+				require_once __DIR__ . '/src/TestListeners/TestListenerDefaultImplementationPHPUnitLte5.php';
+				return;
+			}
+
 			if ( \version_compare( PHPUnit_Version::id(), '7.0.0', '<' ) ) {
-				// PHPUnit 6.4.0 < 7.0.0.
+				// PHPUnit 6.0.0 < 7.0.0.
 				require_once __DIR__ . '/src/TestListeners/TestListenerDefaultImplementationPHPUnit6.php';
 				return;
 			}
diff --git a/src/TestListeners/TestListenerDefaultImplementationPHPUnitLte5.php b/src/TestListeners/TestListenerDefaultImplementationPHPUnitLte5.php
new file mode 100644
index 0000000..a2da65a
--- /dev/null
+++ b/src/TestListeners/TestListenerDefaultImplementationPHPUnitLte5.php
@@ -0,0 +1,155 @@
+<?php
+
+namespace Yoast\PHPUnitPolyfills\TestListeners;
+
+use Exception;
+use PHPUnit_Framework_AssertionFailedError;
+use PHPUnit_Framework_Test;
+use PHPUnit_Framework_TestSuite;
+use PHPUnit_Framework_Warning;
+
+/**
+ * Basic TestListener implementation for use with PHPUnit 5.x.
+ *
+ * This TestListener trait uses renamed (snakecase) methods for all standard methods in
+ * a TestListener to get round the method signature changes in various PHPUnit versions.
+ *
+ * When using this TestListener trait, the snake_case method names need to be used to implement
+ * the listener functionality.
+ *
+ * {@internal While in essence this trait is no different from the PHPUnit 6.x version, this
+ * version is necessary as the class/interface name type declarations used in the PHPUnit 6.x
+ * file are based on the namespaced names. As both the namespaced names as well as the
+ * non-namespaced names exist in PHPUnit 5.7.21+, we cannot create class aliases to
+ * get round the signature mismatch and need this trait using the old names instead.}
+ */
+trait TestListenerDefaultImplementation {
+
+	use TestListenerSnakeCaseMethods;
+
+	/**
+	 * An error occurred.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 * @param Exception              $e    Instance of the error encountered.
+	 * @param float                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addError( PHPUnit_Framework_Test $test, Exception $e, $time ) {
+		$this->add_error( $test, $e, $time );
+	}
+
+	/**
+	 * A warning occurred.
+	 *
+	 * This method is only functional in PHPUnit 6.0 and above.
+	 *
+	 * @param PHPUnit_Framework_Test    $test Test object.
+	 * @param PHPUnit_Framework_Warning $e    Instance of the warning encountered.
+	 * @param float                     $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addWarning( PHPUnit_Framework_Test $test, PHPUnit_Framework_Warning $e, $time ) {
+		$this->add_warning( $test, $e, $time );
+	}
+
+	/**
+	 * A failure occurred.
+	 *
+	 * @param PHPUnit_Framework_Test                 $test Test object.
+	 * @param PHPUnit_Framework_AssertionFailedError $e    Instance of the assertion failure
+	 *                                                     exception encountered.
+	 * @param float                                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addFailure( PHPUnit_Framework_Test $test, PHPUnit_Framework_AssertionFailedError $e, $time ) {
+		$this->add_failure( $test, $e, $time );
+	}
+
+	/**
+	 * Incomplete test.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 * @param Exception              $e    Instance of the incomplete test exception.
+	 * @param float                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addIncompleteTest( PHPUnit_Framework_Test $test, Exception $e, $time ) {
+		$this->add_incomplete_test( $test, $e, $time );
+	}
+
+	/**
+	 * Risky test.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 * @param Exception              $e    Instance of the risky test exception.
+	 * @param float                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addRiskyTest( PHPUnit_Framework_Test $test, Exception $e, $time ) {
+		$this->add_risky_test( $test, $e, $time );
+	}
+
+	/**
+	 * Skipped test.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 * @param Exception              $e    Instance of the skipped test exception.
+	 * @param float                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function addSkippedTest( PHPUnit_Framework_Test $test, Exception $e, $time ) {
+		$this->add_skipped_test( $test, $e, $time );
+	}
+
+	/**
+	 * A test suite started.
+	 *
+	 * @param PHPUnit_Framework_TestSuite $suite Test suite object.
+	 *
+	 * @return void
+	 */
+	public function startTestSuite( PHPUnit_Framework_TestSuite $suite ) {
+		$this->start_test_suite( $suite );
+	}
+
+	/**
+	 * A test suite ended.
+	 *
+	 * @param PHPUnit_Framework_TestSuite $suite Test suite object.
+	 *
+	 * @return void
+	 */
+	public function endTestSuite( PHPUnit_Framework_TestSuite $suite ) {
+		$this->end_test_suite( $suite );
+	}
+
+	/**
+	 * A test started.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 *
+	 * @return void
+	 */
+	public function startTest( PHPUnit_Framework_Test $test ) {
+		$this->start_test( $test );
+	}
+
+	/**
+	 * A test ended.
+	 *
+	 * @param PHPUnit_Framework_Test $test Test object.
+	 * @param float                  $time Execution time of this test.
+	 *
+	 * @return void
+	 */
+	public function endTest( PHPUnit_Framework_Test $test, $time ) {
+		$this->end_test( $test, $time );
+	}
+}
diff --git a/tests/TestListeners/TestListenerTest.php b/tests/TestListeners/TestListenerTest.php
index 5d2969c..9584854 100644
--- a/tests/TestListeners/TestListenerTest.php
+++ b/tests/TestListeners/TestListenerTest.php
@@ -68,8 +68,14 @@ public function testError() {
 	/**
 	 * Test that the TestListener add_warning() method is called.
 	 *
+	 * Note: Prior to PHPUnit 6, PHPUnit did not have `addWarning()` support.
+	 * Interestingly enough, it does seem to work on PHPUnit 5, just don't ask me how.
+	 *
+	 * @requires PHPUnit 5
+	 *
 	 * @return void
 	 */
+	#[RequiresPhpunit( '5' )]
 	public function testWarning() {
 		$test = $this->getTestObject( 'Warning' );
 		$test->run( $this->result );
@@ -110,8 +116,13 @@ public function testIncomplete() {
 	/**
 	 * Test that the TestListener add_risky_test() method is called.
 	 *
+	 * Note: It appears that the PHPUnit native recording of risky tests prior to PHPUnit 6 is buggy.
+	 *
+	 * @requires PHPUnit 6
+	 *
 	 * @return void
 	 */
+	#[RequiresPhpunit( '6' )]
 	public function testRisky() {
 		$test = $this->getTestObject( 'Risky' );
 		$test->run( $this->result );