diff --git a/src/wp-admin/includes/class-wp-site-health-auto-updates.php b/src/wp-admin/includes/class-wp-site-health-auto-updates.php
index f6fb7d79b9..e3c02d0620 100644
--- a/src/wp-admin/includes/class-wp-site-health-auto-updates.php
+++ b/src/wp-admin/includes/class-wp-site-health-auto-updates.php
@@ -277,7 +277,7 @@ class WP_Site_Health_Auto_Updates {
 	public function test_check_wp_filesystem_method() {
 		// Make sure the `request_filesystem_credentials()` function is available during our REST API call.
 		if ( ! function_exists( 'request_filesystem_credentials' ) ) {
-			require_once ABSPATH . '/wp-admin/includes/file.php';
+			require_once ABSPATH . 'wp-admin/includes/file.php';
 		}
 
 		$skin    = new Automatic_Upgrader_Skin;
@@ -328,7 +328,7 @@ class WP_Site_Health_Auto_Updates {
 
 		// Make sure the `get_core_checksums()` function is available during our REST API call.
 		if ( ! function_exists( 'get_core_checksums' ) ) {
-			require_once ABSPATH . '/wp-admin/includes/update.php';
+			require_once ABSPATH . 'wp-admin/includes/update.php';
 		}
 
 		$checksums = get_core_checksums( $wp_version, 'en_US' );
diff --git a/src/wp-admin/includes/class-wp-site-health.php b/src/wp-admin/includes/class-wp-site-health.php
index abe0fc6184..4300abd8de 100644
--- a/src/wp-admin/includes/class-wp-site-health.php
+++ b/src/wp-admin/includes/class-wp-site-health.php
@@ -65,7 +65,7 @@ class WP_Site_Health {
 	 */
 	public function show_site_health_tab( $tab ) {
 		if ( 'debug' === $tab ) {
-			require_once ABSPATH . '/wp-admin/site-health-info.php';
+			require_once ABSPATH . 'wp-admin/site-health-info.php';
 		}
 	}
 
diff --git a/src/wp-admin/includes/file.php b/src/wp-admin/includes/file.php
index bfd32ec899..a89e601060 100644
--- a/src/wp-admin/includes/file.php
+++ b/src/wp-admin/includes/file.php
@@ -2442,7 +2442,7 @@ function request_filesystem_credentials( $form_post, $type = '', $error = false,
 	// Make sure the `submit_button()` function is available during the REST API call
 	// from WP_Site_Health_Auto_Updates::test_check_wp_filesystem_method().
 	if ( ! function_exists( 'submit_button' ) ) {
-		require_once ABSPATH . '/wp-admin/includes/template.php';
+		require_once ABSPATH . 'wp-admin/includes/template.php';
 	}
 	?>
 	<p class="request-filesystem-credentials-action-buttons">
diff --git a/tests/phpunit/includes/bootstrap.php b/tests/phpunit/includes/bootstrap.php
index 9a1b234b71..1113a87eea 100644
--- a/tests/phpunit/includes/bootstrap.php
+++ b/tests/phpunit/includes/bootstrap.php
@@ -302,7 +302,7 @@ if ( isset( $GLOBALS['wp_tests_options'] ) ) {
 }
 
 // Load WordPress.
-require_once ABSPATH . '/wp-settings.php';
+require_once ABSPATH . 'wp-settings.php';
 
 // Delete any default posts & related data.
 _delete_all_posts();
diff --git a/tests/phpunit/includes/install.php b/tests/phpunit/includes/install.php
index 1357b4d5fe..3157fd9384 100644
--- a/tests/phpunit/includes/install.php
+++ b/tests/phpunit/includes/install.php
@@ -37,10 +37,10 @@ $_SERVER['PHP_SELF'] = '/index.php';
 
 tests_add_filter( 'wp_die_handler', '_wp_die_handler_filter_exit' );
 
-require_once ABSPATH . '/wp-settings.php';
+require_once ABSPATH . 'wp-settings.php';
 
-require_once ABSPATH . '/wp-admin/includes/upgrade.php';
-require_once ABSPATH . '/wp-includes/class-wpdb.php';
+require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+require_once ABSPATH . 'wp-includes/class-wpdb.php';
 
 // Override the PHPMailer.
 global $phpmailer;
diff --git a/tests/phpunit/tests/customize/widgets.php b/tests/phpunit/tests/customize/widgets.php
index 446bcf5a68..654008a7a9 100644
--- a/tests/phpunit/tests/customize/widgets.php
+++ b/tests/phpunit/tests/customize/widgets.php
@@ -556,7 +556,7 @@ class Tests_WP_Customize_Widgets extends WP_UnitTestCase {
 	 */
 	private function get_test_widget_control_args() {
 		global $wp_registered_widgets;
-		require_once ABSPATH . '/wp-admin/includes/widgets.php';
+		require_once ABSPATH . 'wp-admin/includes/widgets.php';
 		$widget_id = 'search-2';
 		$widget    = $wp_registered_widgets[ $widget_id ];
 		$args      = array(
diff --git a/tests/phpunit/tests/feed/wpSimplePieFile.php b/tests/phpunit/tests/feed/wpSimplePieFile.php
index f9748e3a7b..fe16352cf7 100644
--- a/tests/phpunit/tests/feed/wpSimplePieFile.php
+++ b/tests/phpunit/tests/feed/wpSimplePieFile.php
@@ -19,8 +19,8 @@ class Tests_Feed_wpSimplePieFile extends WP_UnitTestCase {
 	public static function set_up_before_class() {
 		parent::set_up_before_class();
 
-		require_once ABSPATH . '/wp-includes/class-simplepie.php';
-		require_once ABSPATH . '/wp-includes/class-wp-simplepie-file.php';
+		require_once ABSPATH . 'wp-includes/class-simplepie.php';
+		require_once ABSPATH . 'wp-includes/class-wp-simplepie-file.php';
 	}
 
 	/**
diff --git a/tests/phpunit/tests/pluggable/signatures.php b/tests/phpunit/tests/pluggable/signatures.php
index a802b03766..71c702964a 100644
--- a/tests/phpunit/tests/pluggable/signatures.php
+++ b/tests/phpunit/tests/pluggable/signatures.php
@@ -76,7 +76,7 @@ class Tests_Pluggable_Signatures extends WP_UnitTestCase {
 	 */
 	public function get_defined_pluggable_functions() {
 
-		require_once ABSPATH . '/wp-admin/includes/upgrade.php';
+		require_once ABSPATH . 'wp-admin/includes/upgrade.php';
 
 		$test_functions = array(
 			'install_network',
@@ -103,7 +103,7 @@ class Tests_Pluggable_Signatures extends WP_UnitTestCase {
 		}
 
 		foreach ( $test_files as $file ) {
-			preg_match_all( '#^\t?function (\w+)#m', file_get_contents( ABSPATH . '/' . $file ), $functions );
+			preg_match_all( '#^\t?function (\w+)#m', file_get_contents( ABSPATH . $file ), $functions );
 
 			foreach ( $functions[1] as $function ) {
 				$data[] = array(
diff --git a/tests/phpunit/tests/pluggable/wpMail.php b/tests/phpunit/tests/pluggable/wpMail.php
index bbe27bd88e..68982d6026 100644
--- a/tests/phpunit/tests/pluggable/wpMail.php
+++ b/tests/phpunit/tests/pluggable/wpMail.php
@@ -173,7 +173,7 @@ class Tests_Pluggable_wpMail extends WP_UnitTestCase {
 		$this->assertTrue( wp_mail( 'valid@address.com', 'subject', 'body' ) );
 
 		// Non-fatal errors.
-		$this->assertTrue( wp_mail( 'valid@address.com', 'subject', 'body', "Cc: invalid-address\nBcc: @invalid.address", ABSPATH . '/non-existent-file.html' ) );
+		$this->assertTrue( wp_mail( 'valid@address.com', 'subject', 'body', "Cc: invalid-address\nBcc: @invalid.address", ABSPATH . 'non-existent-file.html' ) );
 
 		// Fatal errors.
 		$this->assertFalse( wp_mail( 'invalid.address', 'subject', 'body', '', array() ) );
diff --git a/tests/phpunit/tests/pomo/po.php b/tests/phpunit/tests/pomo/po.php
index d7652f27e6..8c58099b69 100644
--- a/tests/phpunit/tests/pomo/po.php
+++ b/tests/phpunit/tests/pomo/po.php
@@ -48,7 +48,7 @@ http://wordpress.org/
 	public static function set_up_before_class() {
 		parent::set_up_before_class();
 
-		require_once ABSPATH . '/wp-includes/pomo/po.php';
+		require_once ABSPATH . 'wp-includes/pomo/po.php';
 	}
 
 	public function test_prepend_each_line() {
diff --git a/tests/phpunit/tests/post.php b/tests/phpunit/tests/post.php
index 6f07ca8593..551cbd8c56 100644
--- a/tests/phpunit/tests/post.php
+++ b/tests/phpunit/tests/post.php
@@ -285,7 +285,7 @@ class Tests_Post extends WP_UnitTestCase {
 			$this->markTestSkipped( 'This test is only useful with the utf8 character set.' );
 		}
 
-		require_once ABSPATH . '/wp-admin/includes/post.php';
+		require_once ABSPATH . 'wp-admin/includes/post.php';
 
 		$post_id = self::factory()->post->create();
 
diff --git a/tests/phpunit/tests/widgets.php b/tests/phpunit/tests/widgets.php
index 2407a2de1b..a3dbdfea73 100644
--- a/tests/phpunit/tests/widgets.php
+++ b/tests/phpunit/tests/widgets.php
@@ -813,7 +813,7 @@ class Tests_Widgets extends WP_UnitTestCase {
 		);
 
 		wp_widgets_init();
-		require_once ABSPATH . '/wp-admin/includes/widgets.php';
+		require_once ABSPATH . 'wp-admin/includes/widgets.php';
 		$widget_id    = 'search-2';
 		$widget       = $wp_registered_widgets[ $widget_id ];
 		$params       = array(