if(isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') { // AJAX request } else { // non-AJAX request }This code checks if the "HTTP_X_REQUESTED_WITH" header is set to "XMLHttpRequest", which is a common header set by most AJAX libraries. If the header is set, then the request is considered an AJAX request.