ecshop获取端口的疑问

   if (isset($_SERVER['SERVER_PORT']))
            {
                $port = ':' . $_SERVER['SERVER_PORT'];

                if ((':80' == $port && 'http://' == $protocol) || (':443' == $port && '                          https://' == $protocol))
                {
                    $port = '';
                }
            }

其中

  if ((':80' == $port && 'http://' == $protocol) ||
        (':443' == $port && 'https://' == $protocol))
                {
                    $port = '';
                }

这段代码的意思作用是什么？ 如果条件成立，为什么要把$port 设置为空？

整个函数的代码：

 function get_domain()
    {
        /* 协议 */
        $protocol = $this->http();

        /* 域名或IP地址 */
        if (isset($_SERVER['HTTP_X_FORWARDED_HOST']))
        {
            $host = $_SERVER['HTTP_X_FORWARDED_HOST'];
        }
        elseif (isset($_SERVER['HTTP_HOST']))
        {
            $host = $_SERVER['HTTP_HOST'];
        }
        else
        {
            /* 端口 */
            if (isset($_SERVER['SERVER_PORT']))
            {
                $port = ':' . $_SERVER['SERVER_PORT'];

                if ((':80' == $port && 'http://' == $protocol) || (':443' == $port && 'https://' == $protocol))
                {
                    $port = '';
                }
            }
            else
            {
                $port = '';
            }

            if (isset($_SERVER['SERVER_NAME']))
            {
                $host = $_SERVER['SERVER_NAME'] . $port;
            }
            elseif (isset($_SERVER['SERVER_ADDR']))
            {
                $host = $_SERVER['SERVER_ADDR'] . $port;
            }
        }

        return $protocol . $host;
    }

这段代码你没贴完吧， 完整的似乎是为了我获取当前域名的，如果host不存在的情况下，要port何用。。。