protected function GetBoundary($boundary, $charSet, $contentType, $encoding)
    {
        $result = '';
        if ($charSet == '') {
            $charSet = $this->CharSet;
        }
        if ($contentType == '') {
            $contentType = $this->ContentType;
        }
        if ($encoding == '') {
            $encoding = $this->Encoding;
        }
        $result .= $this->TextLine('--' . $boundary);
        $result .= sprintf("Content-Type: %s; charset=\"%s\"", $contentType, $charSet);
        $result .= $this->LE;
        $result .= $this->HeaderLine('Content-Transfer-Encoding', $encoding);
        $result .= $this->LE;

        return $result;
    }