Prevent direct access to a PHP page

Question

How do I prevent my users from accessing directly pages meant for ajax calls only?

Passing a key during ajax call seems like a solution, whereas access without the k

Answer 1

thanks, albeit I use

define('IS_AJAX', isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');

if(IS_AJAX) {
    //Request identified as ajax request
}

cheers!