php获取真实用户ip

/**
     * 获取用户真实IP
     * @return $onlineIp
     */
    public function getOnlineIp()
    {
        if (getenv('HTTP_CLIENT_IP')) {
            $onlineIp = getenv('HTTP_CLIENT_IP');
        } elseif (getenv('HTTP_X_FORWARDED_FOR')) {
            $onlineIp = getenv('HTTP_X_FORWARDED_FOR');
        } elseif (getenv('REMOTE_ADDR')) {
            $onlineIp = getenv('REMOTE_ADDR');
        } else {
            $onlineIp = 'unknown';
        }
        return $onlineIp;
    }