private function _calculateFileETag($src)
    {
        $src = @is_link($src) ? @readlink($src) : $src;
        $fileStats = stat($src);
        $fileFulltime = @exec('ls --full-time '.escapeshellarg($src));
        $fileMtime = str_pad(preg_match("%\d+\:\d+\:\d+\.(\d+)%", $fileFulltime,
            $fileMtime) ? substr($fileStats['mtime'].$fileMtime[1], 0, 16) : $fileStats['mtime'], 16, '0');
        return sprintf('"%x-%x-%s"', $fileStats['ino'], $fileStats['size'], base_convert($fileMtime, 10, 16));
    }