获取域名

function get_domain() {
    /* 协议 */
    $protocol = (isset($_SERVER['HTTPS']) && (strtolower($_SERVER['HTTPS']) != 'off')) ? 'https://' : '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;
}