AJAX: add a new function, wp_doing_ajax(), which can replace... (wait for it...) DOING_AJAX checks via the constant.

Props Mte90, sebastian.pisula, swissspidy.
Fixes #25669.


git-svn-id: https://develop.svn.wordpress.org/trunk@38334 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/option.php

@@ -786,7 +786,7 @@ function set_transient( $transient, $value, $expiration = 0 ) {
  */
 function wp_user_settings() {
 
-	if ( ! is_admin() || defined( 'DOING_AJAX' ) ) {
+	if ( ! is_admin() || wp_doing_ajax() ) {
 		return;
 	}