php-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;    }