调用代码：
      $unifiedOrder = new WxPayUnifiedOrder();
    $unifiedOrder->SetBody($subject);//商品或支付单简要描述
    $unifiedOrder->SetOut_trade_no($out_trade_no);
    $unifiedOrder->SetTotal_fee($total);
    $unifiedOrder->SetTrade_type("APP");
    $result = WxPayApi::unifiedOrder($unifiedOrder);
    if (is_array($result)) {
        file_put_contents("/tmp/weixin.log",date("Y-m-d H:i:s").":::"."结果：".print_r($result,true)."\n",FILE_APPEND );
        return json_encode($result);
    }

返回结果：
(
    [appid] => 
    [noncestr] => 
    [package] => Sign=WXPay
    [partnerid] => 
    [prepayid] => 
    [timestamp] => 1653546416
    [sign] => D1FD486BB4C67836DD51DA2B8D4C2EBC
)