private function modified($filename)
    {
        if ($this->isSFTP()) {
            $info = @$this->ftp->lstat($filename);
            if ($info) {
                return $info['mtime'];
            }
        } else {
            if ($time = @ftp_mdtm($this->ftp, $filename)) {
                return $time;
            }
        }
        $this->fail("couldn't get the file size for {$filename}");
    }