Coding Standards: Use instanceof keyword instead of the is_a() function.

This is a micro-optimization that removes a few unnecessary function calls.

Follow-up to [31188], [34369], [38986], [41159], [43211], [43230], [44606], [45757].

Props ayeshrajans, jrf, rajinsharwar, costdev, mukesh27, SergeyBiryukov.
Fixes #58943.

git-svn-id: https://develop.svn.wordpress.org/trunk@56352 602fd350-edb4-49c9-b593-d223f7449a82

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();
 		}