private function _addEtag($file) {     $last_modified_time = filemtime($file);     $etag = md5_file($file);     // always send headers     header("Last-Modified: ".gmdate("D, d M Y H:i:s", $last_modified_time)." GMT");     header("Etag: $etag");     // exit if not modified     if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) == $last_modified_time ||     @trim($_SERVER['HTTP_IF_NONE_MATCH']) == $etag) {         header("HTTP/1.1 304 Not Modified");         exit;     } }