src/Message.php
public function getHeaderLines()
    {
        $header = '';

        foreach ($this->getHeaders() as $key => $value)
        {
            // 格式化header的key
            $key = explode('-', $key);
            foreach ($key as $k => $v)
            {
                $key[$k] = ucfirst($v);
            }
            $key = implode('-', $key);

            if (is_array($value))
            {
                $header .= $key . ': ' . (implode(', ', $value));
            }
            else
            {
                $header .= $key . ': ' . $value;
            }
            $header .= "\r\n";
        }

        return $header . "\r\n";
    }
public function getHeaderLines()
    {
        $header = '';

        foreach ($this->getHeaders() as $key => $value)
        {
            // 格式化header的key
            $key = explode('-', $key);
            foreach ($key as $k => $v)
            {
                $key[$k] = ucfirst($v);
            }
            $key = implode('-', $key);

            if (is_array($value))
            {
                $header .= $key . ': ' . (implode(', ', $value));
            }
            else
            {
                $header .= $key . ': ' . $value;
            }
            $header .= "\r\n";
        }

        return $header . "\r\n";
    }
返回一个完整的header

@return string