private function readBoundaryLine($handle, &$lineSeparatorLength = 0)
    {
        $size = 2048;
        $isCut = false;
        $line = fgets($handle, $size);
        while (strlen($line) === $size - 1 && substr($line, -1) !== "\n") {
            $line = fgets($handle, $size);
            $isCut = true;
        }
        $ret = rtrim($line, "\r\n");
        $lineSeparatorLength = strlen($line) - strlen($ret);
        return ($isCut) ? '' : $ret;
    }