- There are changed PHP’s intval(), strval(), floatval(), and boolval() typecasting functions to (int), (string), (float) and (bool) regarding [this doc](https://make.wordpress.org/core/2020/11/20/miscellaneous-developer-focused-changes-in-wordpress-5-6/).

includes/core/class-user.php

@@ -2013,7 +2013,7 @@ if ( ! class_exists( 'um\core\User' ) ) {
 		 *
 		 */
 		function user_exists_by_id( $user_id ) {
-			$aux = get_userdata( intval( $user_id ) );
+			$aux = get_userdata( absint( $user_id ) );
 			if ( $aux == false ) {
 				return false;
 			} else {