Check that request is AJAX

request_is_ajax.php

/**
 * Check if current request is AJAX.
 */
function MODULE_NAME_is_ajax() {
  $http_request = isset($_SERVER['HTTP_X_REQUESTED_WITH'])
  && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest';
  
  // Check ajax in IE.
  $request_uri = isset($_SERVER['REQUEST_URI'])
  && strtolower($_SERVER['REQUEST_URI']) == '/system/ajax';
  
  return $http_request || $request_uri || !empty($_GET['ajax']);
}