官方文档：https://kf.qq.com/faq/1901177NrqMr190117nqYJze.html

    public function  get_weixin_openid()
    {
        //用新的appid获取
        $token = model('Weixin')->get_access_token();
        $url = 'https://api.weixin.qq.com/cgi-bin/changeopenid?access_token='.$token;
        //获取老的openid
        //如果转变过了就is_change改成1
        $userArr = $this->db->table('user')->where('old_openid != '' AND is_change = 0')->order('`uid` desc')->limit(50)->lists();
        //没有了内容
        if(empty($userArr))
        {
            echo "over";
            die;
        }
        $openidArr = [];
        foreach($userArr as $item)
        {
            $openidArr[] = $item['old_openid'];
        }
        $param['from_appid'] = '';//原来微信appid
        $param['openid_list'] = $openidArr;//原app下的openid
        $return = $this->http_post_data($url,json_encode($param));
        $openidNewArr = json_decode($return, TRUE);
        //获取转换的 保存起来即可
        foreach($openidNewArr['result_list'] as $openid)
        {
           if(!empty($openid['err_msg'])&&$openid['err_msg']=='ok')
            {
                if(!empty($openid['new_openid'])){
                $this->db->table('User')->where("old_openid='".$openid['ori_openid']."'")->update(['openid'=>$openid['new_openid']]);
                }
                echo $openid['ori_openid'].'修改为'.$openid['new_openid'].'<br>';
            }
            $this->db->table('User')
                ->where("old_openid='".$openid['ori_openid']."'")
                ->update(['is_change'=>1]);
        }
        echo '<script>location.href="/index.php/weixin/get_weixin_openid"</script>';
    }
    function http_post_data($url, $data_string) {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array(
                'Content-Type: application/json; charset=utf-8',
                'Content-Length: ' . strlen($data_string))
        );
        ob_start();
        curl_exec($ch);
        $return_content = ob_get_contents();
        ob_end_clean();
        $return_code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
        return $return_content;
    }