How to check if the request is an AJAX request with PHP

Example from an archived tutorial:

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

This checks if the HTTP_X_REQUESTED_WITH parameter is not empty and if it’s equal to xmlhttprequest, then it will exit from the script.