function isAjax() {
return (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && 
($_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest'));
}

A simple function to check if the request comes from an AJAX XMLHttpRequest.
Use it if your page should behave differently when called from AJAX.
Comes from jquery mailing list: http://jquery.com/discuss/2006-June/006656/

Check for AJAX Request

ajax, php

PHP