public static function outputXML($string)
 {
     # JSON if no callback
     if (!isset($_GET['callback'])) {
         exit($string);
     }
     $string = str_replace("\n", " ", str_replace("'", '"', $string));
     # JSONP if valid callback
     if (JsonpHelper::is_valid_callback($_GET['callback'])) {
         exit("{$_GET['callback']}('{$string}')");
     }
     # Otherwise, bad request
     header('status: 400 Bad Request', true, 400);
 }