Coding Standards: Use static closures when not using $this.

When a closure does not use `$this`, it can be made `static` for improved performance.

Static closures are supported in PHP since PHP 5.4. ​

Props jrf, hellofromTonya, swissspidy, SergeyBiryukov.
See #53359.

git-svn-id: https://develop.svn.wordpress.org/trunk@51657 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-widgets-controller.php

@@ -792,7 +792,7 @@ class WP_REST_Widgets_Controller extends WP_REST_Controller {
 					'type'        => 'string',
 					'context'     => array(),
 					'arg_options' => array(
-						'sanitize_callback' => function( $string ) {
+						'sanitize_callback' => static function( $string ) {
 							$array = array();
 							wp_parse_str( $string, $array );
 							return $array;