diff --git a/src/wp-content/themes/twentyseventeen/inc/template-tags.php b/src/wp-content/themes/twentyseventeen/inc/template-tags.php
index 437ebd4241..cd0afb7e3e 100644
--- a/src/wp-content/themes/twentyseventeen/inc/template-tags.php
+++ b/src/wp-content/themes/twentyseventeen/inc/template-tags.php
@@ -140,7 +140,7 @@ endif;
  * @param int                  $id      Front page section to display.
  */
 function twentyseventeen_front_page_section( $partial = null, $id = 0 ) {
-	if ( is_a( $partial, 'WP_Customize_Partial' ) ) {
+	if ( $partial instanceof WP_Customize_Partial ) {
 		// Find out the ID and set it up during a selective refresh.
 		global $twentyseventeencounter;
 
diff --git a/src/wp-includes/admin-bar.php b/src/wp-includes/admin-bar.php
index a35051ac8b..d1d438d9e5 100644
--- a/src/wp-includes/admin-bar.php
+++ b/src/wp-includes/admin-bar.php
@@ -897,7 +897,7 @@ function wp_admin_bar_edit_menu( $wp_admin_bar ) {
 					)
 				);
 			}
-		} elseif ( is_a( $current_object, 'WP_User' ) && current_user_can( 'edit_user', $current_object->ID ) ) {
+		} elseif ( $current_object instanceof WP_User && current_user_can( 'edit_user', $current_object->ID ) ) {
 			$edit_user_link = get_edit_user_link( $current_object->ID );
 			if ( $edit_user_link ) {
 				$wp_admin_bar->add_node(
diff --git a/src/wp-includes/http.php b/src/wp-includes/http.php
index c00d1f7e03..e1738beedf 100644
--- a/src/wp-includes/http.php
+++ b/src/wp-includes/http.php
@@ -354,7 +354,7 @@ function wp_remote_retrieve_cookie( $response, $name ) {
 function wp_remote_retrieve_cookie_value( $response, $name ) {
 	$cookie = wp_remote_retrieve_cookie( $response, $name );
 
-	if ( ! is_a( $cookie, 'WP_Http_Cookie' ) ) {
+	if ( ! ( $cookie instanceof WP_Http_Cookie ) ) {
 		return '';
 	}
 
diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index 9b25e49aa9..0ee3a0d615 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -4032,7 +4032,7 @@ function _wp_privacy_account_request_confirmed( $request_id ) {
 function _wp_privacy_send_request_confirmation_notification( $request_id ) {
 	$request = wp_get_user_request( $request_id );
 
-	if ( ! is_a( $request, 'WP_User_Request' ) || 'request-confirmed' !== $request->status ) {
+	if ( ! ( $request instanceof WP_User_Request ) || 'request-confirmed' !== $request->status ) {
 		return;
 	}
 
@@ -4244,7 +4244,7 @@ All at ###SITENAME###
 function _wp_privacy_send_erasure_fulfillment_notification( $request_id ) {
 	$request = wp_get_user_request( $request_id );
 
-	if ( ! is_a( $request, 'WP_User_Request' ) || 'request-completed' !== $request->status ) {
+	if ( ! ( $request instanceof WP_User_Request ) || 'request-completed' !== $request->status ) {
 		return;
 	}
 
diff --git a/src/wp-login.php b/src/wp-login.php
index 8810f5d0d8..3dbfacfd2b 100644
--- a/src/wp-login.php
+++ b/src/wp-login.php
@@ -1303,7 +1303,7 @@ switch ( $action ) {
 			}
 
 			// Check if it is time to add a redirect to the admin email confirmation screen.
-			if ( is_a( $user, 'WP_User' ) && $user->exists() && $user->has_cap( 'manage_options' ) ) {
+			if ( $user instanceof WP_User && $user->exists() && $user->has_cap( 'manage_options' ) ) {
 				$admin_email_lifespan = (int) get_option( 'admin_email_lifespan' );
 
 				/*
diff --git a/tests/phpunit/includes/testcase-rest-api.php b/tests/phpunit/includes/testcase-rest-api.php
index 45e7c5e145..54644f13fb 100644
--- a/tests/phpunit/includes/testcase-rest-api.php
+++ b/tests/phpunit/includes/testcase-rest-api.php
@@ -3,7 +3,7 @@
 abstract class WP_Test_REST_TestCase extends WP_UnitTestCase {
 	protected function assertErrorResponse( $code, $response, $status = null ) {
 
-		if ( is_a( $response, 'WP_REST_Response' ) ) {
+		if ( $response instanceof WP_REST_Response ) {
 			$response = $response->as_error();
 		}
 
diff --git a/tests/phpunit/tests/actions.php b/tests/phpunit/tests/actions.php
index 67222c0819..2f0daaa6a0 100644
--- a/tests/phpunit/tests/actions.php
+++ b/tests/phpunit/tests/actions.php
@@ -535,7 +535,7 @@ class Tests_Actions extends WP_UnitTestCase {
 		foreach ( $hook as $priority => $filter ) {
 			foreach ( $filter as $identifier => $function ) {
 				if ( is_array( $function )
-					&& is_a( $function['function'][0], 'MockAction' )
+					&& $function['function'][0] instanceof MockAction
 					&& 'action' === $function['function'][1]
 				) {
 					remove_filter(