/**
    *  获取用户操作系统
    *
    * @return array $system
    */
    public function get_system ()
    {
        if (strpos($_SERVER['HTTP_USER_AGENT'], 'Android')) {
            return 'Android';
        }elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'iPhone')) {
            return 'IOS';
        } elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'Windows')) {
            return 'windows';
        }else{
            return 'unknown';
        }

    }