How to know request is ajax or other in php

How to know whether the request is an ajax request or normal request in PHP? 

We can know it through following code:

$ajax = false;

if(!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {

 $ajax = true;

}

echo $ajax;