How to get openid for WeChat service account in PHP

    public function tetst(){
      if(!isset($_GET['code'])){
              $APPID = $this->app_id;
              $ran = rand(1,100); //预防缓存
              $REDIRECT_URI = 'http://www.onedayoneclass.com/Home/Index/tetst?number='.$ran.''; //一定写上http://
              $scope='snsapi_userinfo';
              $url='https://open.weixin.qq.com/connect/oauth2/authorize?appid='.$APPID.'&redirect_uri='.urlencode($REDIRECT_URI).'&response_type=code&scope='.$scope.'&state=wx'.'#wechat_redirect';
              //加缓存 随机数
              header("Location:".$url);
          }else{
            $appid = $this->app_id;
            $secret = $this->app_secret;
             $code = $_GET["code"];
             $get_token_url = 'https://api.weixin.qq.com/sns/oauth2/access_token?appid='.$appid.'&secret='.$secret.'&code='.$code.'&grant_type=authorization_code';
             $ch = curl_init();
             curl_setopt($ch,CURLOPT_URL,$get_token_url);
             curl_setopt($ch,CURLOPT_HEADER,0);
             curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
             curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
             $res = curl_exec($ch);
             curl_close($ch);
             $json_obj = json_decode($res,true);
             //根据openid和access_token查询用户信息
             $access_token = $json_obj['access_token'];
             $openid = $json_obj['openid'];
             $get_user_info_url = 'https://api.weixin.qq.com/sns/userinfo?access_token='.$access_token.'&openid='.$openid.'&lang=zh_CN';
             
             $ch = curl_init();
             curl_setopt($ch,CURLOPT_URL,$get_user_info_url);
             curl_setopt($ch,CURLOPT_HEADER,0);
             curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1 );
             curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 10);
             $res = curl_exec($ch);
             curl_close($ch);
             
             //解析json
             $user_obj = json_decode($res,true);
             $_SESSION['user'] = $user_obj;
             print_r($user_obj);
         }
    }