微信V3支付服务端（php）不使用微信SDK

1.获取openid
方法主体

 {  
   $code = '前端传的code';  
    $appid = '你的appid;
    $secret = '你的secret ';
    $api = "https://api.weixin.qq.com/sns/jscode2session?appid={$appid}&secret={$secret}&js_code={$code}&grant_type=authorization_code";
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $api);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
    curl_setopt($ch, CURLOPT_HEADER, 0);
    $output = curl_exec($ch);
    curl_close($ch);
    $json_obj = json_decode($output, true);
    $result = $this->wxPay($json_obj['openid']);
    将$result 返回给前台
}

2.获取prepay_id 并用prepay_id 生成支付sign

   public function wxPay($openid)
    {
    $appid = '你的appid';
    $url = "https://api.mch.weixin.qq.com/v3/pay/transactions/jsapi";
    $nonceStr = $this->getNonceStr();
    $timeStamp = (string)time();
    $out_trade_no = (string)date('YmdHis') . mt_rand(1000,9999);
    $data = array(