Checks if this is an AJAX request

is_ajax.function.php

<?php 

/**
 * Checks if this is an AJAX request
 *
 * @author Luiz "Felds" Liscia <dev@felds.com.br>
 * return bool
 */
function is_ajax()
{
  return isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest';
}