PHP 从数组对象中取出数组提示：Undefined property: stdClass::$subject

是因为PHP增长严谨性，首先要加下判断数组值是否存在，而后在获取！
     if (isset($result->access_token)) {

     }

 $curl = 'https://api.weixin.qq.com/sns/oauth2/access_token?appid='.$this->app_id.'&secret='.$this->app_secret.'&code='.$code.'&grant_type=authorization_code';
        $content = $this->_request($curl);
        $result = json_decode($content);

        //先判断是否有值，而后经过access_token和openid拉取用户信息
        if (isset($result->access_token)) {
            $webAccess_token = $result->access_token;
            $openid = $result->openid;
        }