php获取真实IP地址

function user_realip() {
        if (getenv('HTTP_CLIENT_IP'))
        {
            $ip = getenv('HTTP_CLIENT_IP');
        } elseif (getenv('HTTP_X_FORWARDED_FOR'))
        {
            $ip = getenv('HTTP_X_FORWARDED_FOR');
        } elseif (getenv('REMOTE_ADDR'))
        {
            $ip = getenv('REMOTE_ADDR');
        } else
        {
            $ip = $HTTP_SERVER_VARS['REMOTE_ADDR'];
        }
        return $ip;
    }