public function getHeader($name) {
        if (!is_string($name)) throw new IllegalTypeException('Illegal type of parameter $name: '.gettype($name));
        if (!strlen($name))    throw new InvalidArgumentException('Invalid argument $name: '.$name);

        $headers = $this->getHeaders($name);
        if ($headers)
            return join(', ', $headers);    // combine multiple headers of the same name according to RFC
        return null;
    }