diff --git a/.gitignore b/.gitignore
index 2b59b61..682bfaa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,3 +17,6 @@
 
 # WordPress
 /wordpress/
+
+# PHPUnit
+/.phpunit.result.cache
diff --git a/tests/src/CurrenciesTest.php b/tests/src/CurrenciesTest.php
index 695a266..24a68fc 100644
--- a/tests/src/CurrenciesTest.php
+++ b/tests/src/CurrenciesTest.php
@@ -11,7 +11,7 @@
 namespace Pronamic\WordPress\Money;
 
 use WP_Locale;
-use WP_UnitTestCase;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
 /**
  * Currencies Test
@@ -20,7 +20,7 @@
  * @version 1.2.2
  * @since   1.0.0
  */
-class CurrenciesTest extends WP_UnitTestCase {
+class CurrenciesTest extends TestCase {
 	/**
 	 * Test unknwon currency.
 	 */
diff --git a/tests/src/CurrencyTest.php b/tests/src/CurrencyTest.php
index 4a193c4..b7d98fd 100644
--- a/tests/src/CurrencyTest.php
+++ b/tests/src/CurrencyTest.php
@@ -11,7 +11,7 @@
 namespace Pronamic\WordPress\Money;
 
 use WP_Locale;
-use WP_UnitTestCase;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
 /**
  * Currency Test
@@ -20,7 +20,7 @@
  * @version 1.2.2
  * @since   1.0.0
  */
-class CurrencyTest extends WP_UnitTestCase {
+class CurrencyTest extends TestCase {
 	/**
 	 * Test Euro.
 	 */
diff --git a/tests/src/MoneyTest.php b/tests/src/MoneyTest.php
index 3add8f0..1a36f9c 100644
--- a/tests/src/MoneyTest.php
+++ b/tests/src/MoneyTest.php
@@ -11,7 +11,7 @@
 namespace Pronamic\WordPress\Money;
 
 use WP_Locale;
-use WP_UnitTestCase;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
 /**
  * Money
@@ -20,12 +20,12 @@
  * @version 1.2.2
  * @since   1.0.0
  */
-class MoneyTest extends WP_UnitTestCase {
+class MoneyTest extends TestCase {
 	/**
 	 * Setup.
 	 */
-	public function setUp() {
-		parent::setUp();
+	public function set_up() {
+		parent::set_up();
 
 		if ( version_compare( PHP_VERSION, '5.4', '<' ) ) {
 			add_filter( 'number_format_i18n', [ $this, 'maybe_fix_multibyte_number_format' ], 10, 3 );
diff --git a/tests/src/ParserTest.php b/tests/src/ParserTest.php
index 0077168..b981b5e 100644
--- a/tests/src/ParserTest.php
+++ b/tests/src/ParserTest.php
@@ -10,7 +10,7 @@
 
 namespace Pronamic\WordPress\Money;
 
-use WP_UnitTestCase;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
 /**
  * Parser
@@ -19,7 +19,7 @@
  * @version 1.2.5
  * @since   1.1.0
  */
-class ParserTest extends WP_UnitTestCase {
+class ParserTest extends TestCase {
 	/**
 	 * Parser.
 	 *
@@ -30,8 +30,8 @@ class ParserTest extends WP_UnitTestCase {
 	/**
 	 * Setup.
 	 */
-	public function setUp() {
-		parent::setUp();
+	public function set_up() {
+		parent::set_up();
 
 		$this->parser = new Parser();
 	}
diff --git a/tests/src/TaxedMoneyTest.php b/tests/src/TaxedMoneyTest.php
index a7d360c..1438d39 100644
--- a/tests/src/TaxedMoneyTest.php
+++ b/tests/src/TaxedMoneyTest.php
@@ -11,7 +11,7 @@
 namespace Pronamic\WordPress\Money;
 
 use WP_Locale;
-use WP_UnitTestCase;
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
 
 /**
  * Taxed Money Test
@@ -20,7 +20,7 @@
  * @version 1.2.2
  * @since   1.0.0
  */
-class TaxedMoneyTest extends WP_UnitTestCase {
+class TaxedMoneyTest extends TestCase {
 	/**
 	 *  Test taxed money.
 	 */