diff --git a/.gitignore b/.gitignore
index f252a70c5..535811fa5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,3 +8,6 @@ tests/coverage/*
 # Ignore local overrides of the PHPCS config file.
 .phpcs.xml
 phpcs.xml
+
+# Ignore PHPUnit results cache file.
+.phpunit.result.cache
diff --git a/composer.json b/composer.json
index 2e051c9e8..1b61196b2 100644
--- a/composer.json
+++ b/composer.json
@@ -40,13 +40,13 @@
   },
   "require-dev": {
     "requests/test-server": "dev-master",
-    "phpunit/phpunit": "^5.7 || ^6.5 || ^7.5",
     "squizlabs/php_codesniffer": "^3.5",
     "phpcompatibility/php-compatibility": "^9.0",
     "wp-coding-standards/wpcs": "^2.0",
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7",
     "php-parallel-lint/php-parallel-lint": "^1.3",
-    "php-parallel-lint/php-console-highlighter": "^0.5.0"
+    "php-parallel-lint/php-console-highlighter": "^0.5.0",
+    "yoast/phpunit-polyfills": "^1.0.0"
   },
   "type": "library",
   "autoload": {
diff --git a/tests/Auth/Basic.php b/tests/Auth/Basic.php
index eec54cfc1..eff132613 100644
--- a/tests/Auth/Basic.php
+++ b/tests/Auth/Basic.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Auth_Basic extends PHPUnit_Framework_TestCase {
+class RequestsTest_Auth_Basic extends RequestsTest_TestCase {
 	public static function transportProvider() {
 		return array(
 			array('Requests_Transport_fsockopen'),
@@ -76,11 +76,9 @@ public function testPOSTUsingInstantiation($transport) {
 		$this->assertSame('test', $result->data);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid number of arguments
-	 */
 	public function testMissingPassword() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid number of arguments');
 		new Requests_Auth_Basic(array('user'));
 	}
 
diff --git a/tests/ChunkedEncoding.php b/tests/ChunkedEncoding.php
index f3ca0a053..e1c03aad7 100644
--- a/tests/ChunkedEncoding.php
+++ b/tests/ChunkedEncoding.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_ChunkedDecoding extends PHPUnit_Framework_TestCase {
+class RequestsTest_ChunkedDecoding extends RequestsTest_TestCase {
 	public static function chunkedProvider() {
 		return array(
 			array(
diff --git a/tests/Cookies.php b/tests/Cookies.php
index f0f660104..03c6aa5ea 100644
--- a/tests/Cookies.php
+++ b/tests/Cookies.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Cookies extends PHPUnit_Framework_TestCase {
+class RequestsTest_Cookies extends RequestsTest_TestCase {
 	public function testBasicCookie() {
 		$cookie = new Requests_Cookie('requests-testcookie', 'testvalue');
 
@@ -57,11 +57,9 @@ public function testCookieJarUnsetter() {
 		$this->assertFalse(isset($jar['requests-testcookie']));
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Object is a dictionary, not a list
-	 */
 	public function testCookieJarAsList() {
+		$this->expectException(Requests_Exception::class);
+		$this->expectExceptionMessage('Object is a dictionary, not a list');
 		$cookies   = new Requests_Cookie_Jar();
 		$cookies[] = 'requests-testcookie1=testvalue1';
 	}
@@ -111,7 +109,7 @@ protected function setCookieRequest($cookies) {
 		$response = Requests::get(httpbin('/cookies/set'), array(), $options);
 
 		$data = json_decode($response->body, true);
-		$this->assertInternalType('array', $data);
+		$this->assertIsArray($data);
 		$this->assertArrayHasKey('cookies', $data);
 		return $data['cookies'];
 	}
diff --git a/tests/Encoding.php b/tests/Encoding.php
index 54dc5ec8a..800fddcf9 100644
--- a/tests/Encoding.php
+++ b/tests/Encoding.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTests_Encoding extends PHPUnit_Framework_TestCase {
+class RequestsTests_Encoding extends RequestsTest_TestCase {
 	protected static function mapData($type, $data) {
 		$real_data = array();
 		foreach ($data as $value) {
@@ -84,11 +84,4 @@ public function testCompatibleInflate($original, $encoded) {
 		$decoded = Requests::compatible_gzinflate($encoded);
 		$this->assertSame($original, $decoded);
 	}
-
-	protected function bin2hex($field) {
-		$field = bin2hex($field);
-		$field = chunk_split($field, 2, "\\x");
-		$field = "\\x" . substr($field, 0, -2);
-		return $field;
-	}
 }
diff --git a/tests/IDNAEncoder.php b/tests/IDNAEncoder.php
index dc3f37e53..277fc5e9d 100644
--- a/tests/IDNAEncoder.php
+++ b/tests/IDNAEncoder.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_IDNAEncoder extends PHPUnit_Framework_TestCase {
+class RequestsTest_IDNAEncoder extends RequestsTest_TestCase {
 	public static function specExamples() {
 		return array(
 			array(
@@ -22,29 +22,23 @@ public function testEncoding($data, $expected) {
 		$this->assertSame($expected, $result);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Provided string is too long
-	 */
 	public function testASCIITooLong() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Provided string is too long');
 		$data = str_repeat('abcd', 20);
 		Requests_IDNAEncoder::encode($data);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Encoded string is too long
-	 */
 	public function testEncodedTooLong() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Encoded string is too long');
 		$data = str_repeat("\xe4\xbb\x96", 60);
 		Requests_IDNAEncoder::encode($data);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Provided string begins with ACE prefix
-	 */
 	public function testAlreadyPrefixed() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Provided string begins with ACE prefix');
 		Requests_IDNAEncoder::encode("xn--\xe4\xbb\x96");
 	}
 
@@ -68,43 +62,33 @@ public function testFourByteCharacter() {
 		$this->assertSame('xn--ww6j', $result);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid Unicode codepoint
-	 */
 	public function testFiveByteCharacter() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid Unicode codepoint');
 		Requests_IDNAEncoder::encode("\xfb\xb6\xb6\xb6\xb6");
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid Unicode codepoint
-	 */
 	public function testSixByteCharacter() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid Unicode codepoint');
 		Requests_IDNAEncoder::encode("\xfd\xb6\xb6\xb6\xb6\xb6");
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid Unicode codepoint
-	 */
 	public function testInvalidASCIICharacterWithMultibyte() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid Unicode codepoint');
 		Requests_IDNAEncoder::encode("\0\xc2\xb6");
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid Unicode codepoint
-	 */
 	public function testUnfinishedMultibyte() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid Unicode codepoint');
 		Requests_IDNAEncoder::encode("\xc2");
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid Unicode codepoint
-	 */
 	public function testPartialMultibyte() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid Unicode codepoint');
 		Requests_IDNAEncoder::encode("\xc2\xc2\xb6");
 	}
 }
diff --git a/tests/IRI.php b/tests/IRI.php
index 488083fc1..cef75078f 100644
--- a/tests/IRI.php
+++ b/tests/IRI.php
@@ -40,7 +40,7 @@
  *
  */
 
-class RequestsTest_IRI extends PHPUnit_Framework_TestCase
+class RequestsTest_IRI extends RequestsTest_TestCase
 {
 	public static function rfc3986_tests()
 	{
@@ -384,11 +384,9 @@ public function testWriteAliased()
 		$this->assertSame('test', $iri->fragment);
 	}
 
-	/**
-	 * @expectedException PHPUnit_Framework_Error_Notice
-	 */
 	public function testNonexistantProperty()
 	{
+		$this->expectNotice('Undefined property: Requests_IRI::nonexistant_prop');
 		$iri = new Requests_IRI();
 		$this->assertFalse(isset($iri->nonexistant_prop));
 		$should_fail = $iri->nonexistant_prop;
diff --git a/tests/Proxy/HTTP.php b/tests/Proxy/HTTP.php
index 91a8350ff..a6de95deb 100644
--- a/tests/Proxy/HTTP.php
+++ b/tests/Proxy/HTTP.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Proxy_HTTP extends PHPUnit_Framework_TestCase {
+class RequestsTest_Proxy_HTTP extends RequestsTest_TestCase {
 	protected function checkProxyAvailable($type = '') {
 		switch ($type) {
 			case 'auth':
@@ -60,9 +60,6 @@ public function testConnectWithArray($transport) {
 
 	/**
 	 * @dataProvider transportProvider
-	 *
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Invalid number of arguments
 	 */
 	public function testConnectInvalidParameters($transport) {
 		$this->checkProxyAvailable();
@@ -71,6 +68,8 @@ public function testConnectInvalidParameters($transport) {
 			'proxy'     => array(REQUESTS_HTTP_PROXY, 'testuser', 'password', 'something'),
 			'transport' => $transport,
 		);
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Invalid number of arguments');
 		Requests::get(httpbin('/get'), array(), $options);
 	}
 
diff --git a/tests/Requests.php b/tests/Requests.php
index 49eee86f0..8e04e1465 100644
--- a/tests/Requests.php
+++ b/tests/Requests.php
@@ -1,11 +1,10 @@
 <?php
 
-class RequestsTest_Requests extends PHPUnit_Framework_TestCase {
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Only HTTP(S) requests are handled
-	 */
+class RequestsTest_Requests extends RequestsTest_TestCase {
+
 	public function testInvalidProtocol() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Only HTTP(S) requests are handled');
 		Requests::request('ftp://128.0.0.1/');
 	}
 
@@ -100,9 +99,6 @@ public function testHeaderOnlyLF() {
 	 *
 	 * We do not support HTTP/0.9. If this is really an issue for you, file a
 	 * new issue, and update your server/proxy to support a proper protocol.
-	 *
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Response could not be parsed
 	 */
 	public function testInvalidProtocolVersion() {
 		$transport       = new RequestsTest_Mock_RawTransport();
@@ -111,14 +107,14 @@ public function testInvalidProtocolVersion() {
 		$options = array(
 			'transport' => $transport,
 		);
+
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Response could not be parsed');
 		Requests::get('http://example.com/', array(), $options);
 	}
 
 	/**
-	 * HTTP/0.9 also appears to use a single CRLF instead of two
-	 *
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Missing header/body separator
+	 * HTTP/0.9 also appears to use a single CRLF instead of two.
 	 */
 	public function testSingleCRLFSeparator() {
 		$transport       = new RequestsTest_Mock_RawTransport();
@@ -127,13 +123,12 @@ public function testSingleCRLFSeparator() {
 		$options = array(
 			'transport' => $transport,
 		);
+
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Missing header/body separator');
 		Requests::get('http://example.com/', array(), $options);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Response could not be parsed
-	 */
 	public function testInvalidStatus() {
 		$transport       = new RequestsTest_Mock_RawTransport();
 		$transport->data = "HTTP/1.1 OK\r\nTest: value\nAnother-Test: value\r\n\r\nTest";
@@ -141,6 +136,9 @@ public function testInvalidStatus() {
 		$options = array(
 			'transport' => $transport,
 		);
+
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Response could not be parsed');
 		Requests::get('http://example.com/', array(), $options);
 	}
 
@@ -156,12 +154,10 @@ public function test30xWithoutLocation() {
 		$this->assertSame(0, $response->redirects);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage timed out
-	 */
 	public function testTimeoutException() {
 		$options = array('timeout' => 0.5);
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('timed out');
 		Requests::get(httpbin('/delay/3'), array(), $options);
 	}
 }
diff --git a/tests/Response/Headers.php b/tests/Response/Headers.php
index b5fbe2a76..65f2b838b 100644
--- a/tests/Response/Headers.php
+++ b/tests/Response/Headers.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Response_Headers extends PHPUnit_Framework_TestCase {
+class RequestsTest_Response_Headers extends RequestsTest_TestCase {
 	public function testArrayAccess() {
 		$headers                 = new Requests_Response_Headers();
 		$headers['Content-Type'] = 'text/plain';
@@ -37,11 +37,9 @@ public function testIteration() {
 		}
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Object is a dictionary, not a list
-	 */
 	public function testInvalidKey() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Object is a dictionary, not a list');
 		$headers   = new Requests_Response_Headers();
 		$headers[] = 'text/plain';
 	}
diff --git a/tests/SSL.php b/tests/SSL.php
index 8c28c1d55..a23ccd696 100644
--- a/tests/SSL.php
+++ b/tests/SSL.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_SSL extends PHPUnit_Framework_TestCase {
+class RequestsTest_SSL extends RequestsTest_TestCase {
 	public static function domainMatchProvider() {
 		return array(
 			array('example.com', 'example.com'),
diff --git a/tests/Session.php b/tests/Session.php
index 7db0cb2d6..b8e465dde 100644
--- a/tests/Session.php
+++ b/tests/Session.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Session extends PHPUnit_Framework_TestCase {
+class RequestsTest_Session extends RequestsTest_TestCase {
 	public function testURLResolution() {
 		$session = new Requests_Session(httpbin('/'));
 
diff --git a/tests/TestCase.php b/tests/TestCase.php
new file mode 100644
index 000000000..f464f86ed
--- /dev/null
+++ b/tests/TestCase.php
@@ -0,0 +1,7 @@
+<?php
+
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
+
+class RequestsTest_TestCase extends TestCase {
+
+}
diff --git a/tests/Transport/Base.php b/tests/Transport/Base.php
index facedd4e2..066c5ff5a 100644
--- a/tests/Transport/Base.php
+++ b/tests/Transport/Base.php
@@ -1,7 +1,7 @@
 <?php
 
-abstract class RequestsTest_Transport_Base extends PHPUnit_Framework_TestCase {
-	public function setUp() {
+abstract class RequestsTest_Transport_Base extends RequestsTest_TestCase {
+	public function set_up() {
 		$callback  = array($this->transport, 'test');
 		$supported = call_user_func($callback);
 
@@ -17,29 +17,6 @@ public function setUp() {
 	}
 	protected $skip_https = false;
 
-	/**
-	 * PHPUnit 6+ compatibility shim.
-	 *
-	 * @param mixed      $exception
-	 * @param string     $message
-	 * @param int|string $code
-	 */
-	public function setExpectedException($exception, $message = '', $code = null) {
-		if (method_exists('PHPUnit_Framework_TestCase', 'setExpectedException')) {
-			parent::setExpectedException($exception, $message, $code);
-		}
-		else {
-			$this->expectException($exception);
-			if ($message !== null) {
-				$this->expectExceptionMessage($message);
-			}
-			if ($code !== null) {
-				$this->expectExceptionCode($code);
-			}
-		}
-	}
-
-
 	protected function getOptions($other = array()) {
 		$options = array(
 			'transport' => $this->transport,
@@ -344,14 +321,12 @@ public function testRelativeRedirects() {
 		$this->assertSame(6, $request->redirects);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage Too many redirects
-	 */
 	public function testTooManyRedirects() {
 		$options = array(
 			'redirects' => 10, // default, but force just in case
 		);
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('Too many redirects');
 		Requests::get(httpbin('/redirect/11'), array(), $this->getOptions($options));
 	}
 
@@ -437,10 +412,11 @@ public function testStatusCodeThrow($code, $success) {
 
 		if (!$success) {
 			if ($code >= 400) {
-				$this->setExpectedException('Requests_Exception_HTTP_' . $code, null, $code);
+				$this->expectException('Requests_Exception_HTTP_' . $code);
+				$this->expectExceptionCode($code);
 			}
 			elseif ($code >= 300 && $code < 400) {
-				$this->setExpectedException('Requests_Exception', null);
+				$this->expectException('Requests_Exception');
 			}
 		}
 
@@ -466,7 +442,8 @@ public function testStatusCodeThrowAllowRedirects($code, $success) {
 
 		if (!$success) {
 			if ($code >= 400 || $code === 304 || $code === 305 || $code === 306) {
-				$this->setExpectedException('Requests_Exception_HTTP_' . $code, null, $code);
+				$this->expectException('Requests_Exception_HTTP_' . $code);
+				$this->expectExceptionCode($code);
 			}
 		}
 
@@ -490,10 +467,6 @@ public function testStatusCodeUnknown() {
 		$this->assertFalse($request->success);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception_HTTP_Unknown
-	 * @expectedExceptionMessage 599 Unknown
-	 */
 	public function testStatusCodeThrowUnknown() {
 		$transport       = new RequestsTest_Mock_Transport();
 		$transport->code = 599;
@@ -503,6 +476,8 @@ public function testStatusCodeThrowUnknown() {
 		);
 
 		$request = Requests::get(httpbin('/status/599'), array(), $options);
+		$this->expectException('Requests_Exception_HTTP_Unknown');
+		$this->expectExceptionMessage('599 Unknown');
 		$request->throw_for_status(true);
 	}
 
@@ -562,10 +537,8 @@ public function testNonblocking() {
 		$this->assertEquals($empty, $request);
 	}
 
-	/**
-	 * @expectedException Requests_Exception
-	 */
 	public function testBadIP() {
+		$this->expectException('Requests_Exception');
 		Requests::get('http://256.256.256.0/', array(), $this->getOptions());
 	}
 
@@ -582,34 +555,28 @@ public function testHTTPS() {
 		$this->assertEmpty($result['args']);
 	}
 
-	/**
-	 * @expectedException Requests_Exception
-	 */
 	public function testExpiredHTTPS() {
 		if ($this->skip_https) {
 			$this->markTestSkipped('SSL support is not available.');
 			return;
 		}
 
+		$this->expectException('Requests_Exception');
 		Requests::get('https://testssl-expire.disig.sk/index.en.html', array(), $this->getOptions());
 	}
 
-	/**
-	 * @expectedException Requests_Exception
-	 */
 	public function testRevokedHTTPS() {
 		if ($this->skip_https) {
 			$this->markTestSkipped('SSL support is not available.');
 			return;
 		}
 
+		$this->expectException('Requests_Exception');
 		Requests::get('https://testssl-revoked.disig.sk/index.en.html', array(), $this->getOptions());
 	}
 
 	/**
 	 * Test that SSL fails with a bad certificate
-	 *
-	 * @expectedException Requests_Exception
 	 */
 	public function testBadDomain() {
 		if ($this->skip_https) {
@@ -617,6 +584,7 @@ public function testBadDomain() {
 			return;
 		}
 
+		$this->expectException('Requests_Exception');
 		Requests::head('https://wrong.host.badssl.com/', array(), $this->getOptions());
 	}
 
@@ -663,14 +631,12 @@ public function testSNISupport() {
 		$this->assertSame(200, $request->status_code);
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage timed out
-	 */
 	public function testTimeout() {
 		$options = array(
 			'timeout' => 1,
 		);
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('timed out');
 		Requests::get(httpbin('/delay/10'), array(), $this->getOptions($options));
 	}
 
@@ -850,13 +816,6 @@ public function testAlternatePort() {
 		$this->assertSame('8080', $matches[1]);
 	}
 
-	/**
-	 * This test will be skipped on PHP 8 as it would fail due to the use of an incompatible
-	 * PHPUnit version. Once the test suite is compatible with PHPUnit 9, this "requires" can
-	 * be removed.
-	 *
-	 * @requires PHP < 8
-	 */
 	public function testProgressCallback() {
 		$mock = $this->getMockBuilder('stdClass')->setMethods(array('progress'))->getMock();
 		$mock->expects($this->atLeastOnce())->method('progress');
@@ -870,13 +829,6 @@ public function testProgressCallback() {
 		Requests::get(httpbin('/get'), array(), $options);
 	}
 
-	/**
-	 * This test will be skipped on PHP 8 as it would fail due to the use of an incompatible
-	 * PHPUnit version. Once the test suite is compatible with PHPUnit 9, this "requires" can
-	 * be removed.
-	 *
-	 * @requires PHP < 8
-	 */
 	public function testAfterRequestCallback() {
 		$mock = $this->getMockBuilder('stdClass')
 			->setMethods(array('after_request'))
diff --git a/tests/Transport/cURL.php b/tests/Transport/cURL.php
index 0690d7b4b..7f7c41774 100644
--- a/tests/Transport/cURL.php
+++ b/tests/Transport/cURL.php
@@ -3,37 +3,30 @@
 class RequestsTest_Transport_cURL extends RequestsTest_Transport_Base {
 	protected $transport = 'Requests_Transport_cURL';
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage t resolve host
-	 */
 	public function testBadIP() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('t resolve host');
 		parent::testBadIP();
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage certificate subject name
-	 */
 	public function testExpiredHTTPS() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('certificate subject name');
 		parent::testExpiredHTTPS();
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage certificate subject name
-	 */
 	public function testRevokedHTTPS() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('certificate subject name');
 		parent::testRevokedHTTPS();
 	}
 
 	/**
 	 * Test that SSL fails with a bad certificate
-	 *
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage certificate subject name
 	 */
 	public function testBadDomain() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('certificate subject name');
 		parent::testBadDomain();
 	}
 
diff --git a/tests/Transport/fsockopen.php b/tests/Transport/fsockopen.php
index 21113e6bf..f77c154a1 100644
--- a/tests/Transport/fsockopen.php
+++ b/tests/Transport/fsockopen.php
@@ -3,36 +3,29 @@
 class RequestsTest_Transport_fsockopen extends RequestsTest_Transport_Base {
 	protected $transport = 'Requests_Transport_fsockopen';
 
-	/**
-	 * @expectedException Requests_Exception
-	 */
 	public function testBadIP() {
+		$this->expectException('Requests_Exception');
 		parent::testBadIP();
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage SSL certificate did not match the requested domain name
-	 */
 	public function testExpiredHTTPS() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('SSL certificate did not match the requested domain name');
 		parent::testExpiredHTTPS();
 	}
 
-	/**
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage SSL certificate did not match the requested domain name
-	 */
 	public function testRevokedHTTPS() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('SSL certificate did not match the requested domain name');
 		parent::testRevokedHTTPS();
 	}
 
 	/**
 	 * Test that SSL fails with a bad certificate
-	 *
-	 * @expectedException        Requests_Exception
-	 * @expectedExceptionMessage SSL certificate did not match the requested domain name
 	 */
 	public function testBadDomain() {
+		$this->expectException('Requests_Exception');
+		$this->expectExceptionMessage('SSL certificate did not match the requested domain name');
 		parent::testBadDomain();
 	}
 
@@ -50,6 +43,6 @@ public function testContentLengthHeader() {
 	 * Issue #248.
 	 */
 	public function checkContentLengthHeader($headers) {
-		$this->assertContains('Content-Length: 0', $headers);
+		$this->assertStringContainsString('Content-Length: 0', $headers);
 	}
 }
diff --git a/tests/Utility/FilteredIterator.php b/tests/Utility/FilteredIterator.php
index 812ed087f..01f9e0a2b 100644
--- a/tests/Utility/FilteredIterator.php
+++ b/tests/Utility/FilteredIterator.php
@@ -1,6 +1,6 @@
 <?php
 
-class RequestsTest_Utility_FilteredIterator extends PHPUnit_Framework_TestCase {
+class RequestsTest_Utility_FilteredIterator extends RequestsTest_TestCase {
 	/**
 	 * @dataProvider dataSerializeDeserializeObjects
 	 */
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index f94a4872f..10aa38155 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -1,12 +1,5 @@
 <?php
 
-/**
- * Compatibility with PHPUnit 6+
- */
-if (class_exists('PHPUnit\Runner\Version')) {
-	require_once dirname(__FILE__) . '/phpunit6-compat.php';
-}
-
 date_default_timezone_set('UTC');
 
 function define_from_env($name, $default = false) {
@@ -28,9 +21,17 @@ function define_from_env($name, $default = false) {
 define_from_env('REQUESTS_HTTP_PROXY_AUTH_USER');
 define_from_env('REQUESTS_HTTP_PROXY_AUTH_PASS');
 
-require_once dirname(dirname(__FILE__)) . '/library/Requests.php';
+require_once dirname(__DIR__) . '/library/Requests.php';
 Requests::register_autoloader();
 
+$polyfill_autoloader = dirname(__DIR__) . '/vendor/yoast/phpunit-polyfills/phpunitpolyfills-autoload.php';
+if (file_exists($polyfill_autoloader)) {
+	require_once $polyfill_autoloader;
+} else {
+	echo 'Please run `composer install` before attempting to run the tests.', PHP_EOL;
+	die(1);
+}
+
 function autoload_tests($class) {
 	if (strpos($class, 'RequestsTest_') !== 0) {
 		return;
@@ -38,8 +39,8 @@ function autoload_tests($class) {
 
 	$class = substr($class, 13);
 	$file  = str_replace('_', '/', $class);
-	if (file_exists(dirname(__FILE__) . '/' . $file . '.php')) {
-		require_once dirname(__FILE__) . '/' . $file . '.php';
+	if (file_exists(__DIR__ . '/' . $file . '.php')) {
+		require_once __DIR__ . '/' . $file . '.php';
 	}
 }
 
diff --git a/tests/phpunit6-compat.php b/tests/phpunit6-compat.php
deleted file mode 100644
index 8eb119583..000000000
--- a/tests/phpunit6-compat.php
+++ /dev/null
@@ -1,16 +0,0 @@
-<?php
-
-if (class_exists('PHPUnit\Runner\Version') && version_compare(PHPUnit\Runner\Version::id(), '6.0', '>=')) {
-	class_alias('PHPUnit\Framework\TestCase', 'PHPUnit_Framework_TestCase');
-	class_alias('PHPUnit\Framework\Exception', 'PHPUnit_Framework_Exception');
-	class_alias('PHPUnit\Framework\ExpectationFailedException', 'PHPUnit_Framework_ExpectationFailedException');
-	class_alias('PHPUnit\Framework\Error\Notice', 'PHPUnit_Framework_Error_Notice');
-	class_alias('PHPUnit\Framework\Error\Warning', 'PHPUnit_Framework_Error_Warning');
-	class_alias('PHPUnit\Framework\Test', 'PHPUnit_Framework_Test');
-	class_alias('PHPUnit\Framework\Warning', 'PHPUnit_Framework_Warning');
-	class_alias('PHPUnit\Framework\AssertionFailedError', 'PHPUnit_Framework_AssertionFailedError');
-	class_alias('PHPUnit\Framework\TestSuite', 'PHPUnit_Framework_TestSuite');
-	class_alias('PHPUnit\Framework\TestListener', 'PHPUnit_Framework_TestListener');
-	class_alias('PHPUnit\Util\GlobalState', 'PHPUnit_Util_GlobalState');
-	class_alias('PHPUnit\Util\Getopt', 'PHPUnit_Util_Getopt');
-}