请先看微信支付退款通知文档【https://pay.weixin.qq.com/wiki/doc/apiv3/apis/chapter3_5_11.shtml】

再看这个【https://github.com/lampnick/wxpay-refund-notify-decrypt-helper】

wxPaymentNotice VerifySign 微信支付结果通知

wxPayRefundNotice xmlToArray 微信支付退款结果通知

PHP代码

<?php

namespace App\Http\Controllers\WxPay;

use Illuminate\Http\Request;
use App\Http\Controllers\Controller;
use WechatPay\GuzzleMiddleware\Util\AesUtil;

class WxPayNotice extends Controller
{
    //    微信支付结果通知处理
    public function wxPaymentNotice(Request $request)
    {
        $this->validate($request, [
            'id' => 'bail|required|string|min:1',
            'create_time' => 'required|string|min:1',
            'event_type' => 'required|string|min:1',
            'resource_type' => 'required|string|min:1',
            'resource.algorithm' => 'required|string|min:1',
            'resource.ciphertext' => 'required|string|min:1',
            'resource.associated_data' => 'sometimes|required|string',
            'resource.original_type' => 'required|string|min:1',
            'resource.nonce' => 'required|string|min:1',
            'summary' => 'required|string|min:1'
        ]);
        $contents = $request['resource'];
        $signature = $request->header("Wechatpay-Signature");
        $timestamp = $request->header("Wechatpay-Timestamp");
        $serial = $request->header("Wechatpay-Serial");
        $nonce = $request->header("Wechatpay-Nonce");
        $params = file_get_contents('php://input');
        $bf_msg = $timestamp . "\n" . $nonce . "\n" . $params . "\n";

        $outpath = '日志(看需求).txt';
        $handle = fopen($outpath, 'ab+');
        if($serial!==config('globaldatas.wechat_no')) {
            fwrite($handle, "签名证书序列号不一致" . PHP_EOL);
        }
        $check_sign = $this->VerifySign($bf_msg, $signature);
        $plainCerts='"check_sign":"' . $check_sign . PHP_EOL;
        fwrite($handle, $plainCerts);
        $apiv = config('globaldatas.apiv');
        $decrypter = new AesUtil($apiv);
        $plain = $decrypter->decryptToString($contents['associated_data'], $contents['nonce'], $contents['ciphertext']);
        if (!$plain) {
            fwrite($handle, "encrypted certificate decrypt fail!" . PHP_EOL);
            fclose($handle);
            exit(1);
        }
        $plainCerts = '"id":' . json_encode($plain) . PHP_EOL;
        fwrite($handle, $plainCerts);
        fclose($handle);
//        return [
//            "code"=> "ERROR_NAME", //SUCCESS
//            "message"=> "ERROR_DESCRIPTION"
//        ];
//        return [
//            "code"=> "SUCCESS",
//            "message"=> "成功收到微信支付结果通知并解密",
//            "ret"=>$plain
//        ];
    }
//    检验微信支付结果通知签名
    public function VerifySign($data, $signature)
    {
        $signature = base64_decode($signature);//解密应答签名
        // 微信支付平台证书
        $wechat_no = '微信支付平台证书.pem';
        $wechatpayCertificate = openssl_pkey_get_public(file_get_contents($wechat_no));
        $retCode = openssl_verify($data, $signature, $wechatpayCertificate, OPENSSL_ALGO_SHA256);
        if ($retCode == 1) {
            return true;
        }
        return false;
    }
//    微信支付退款结果通知
    public function wxPayRefundNotice(Request $request) {
        $param = file_get_contents('php://input');
        $valueArr=static::xmlToArray($param);
        $req_info=$valueArr['req_info'];

        $key=config('globaldatas.apivv');
        $md5LowerKey = strtolower(md5($key));
        $decrypted = openssl_decrypt($req_info, "AES-256-ECB", $md5LowerKey);
        $decryptedData =static::xmlToArray($decrypted);

        $printStr=json_encode($decryptedData).PHP_EOL;
        $outpath = '日志(看需求).txt';
        file_put_contents($outpath,$printStr, FILE_APPEND | LOCK_EX);

        return $decryptedData;
    }
    public static function xmlToArray($xml) {
        $valueXml = simplexml_load_string($xml, 'SimpleXMLElement', LIBXML_NOCDATA);
        $valueObj = json_encode($valueXml);
        $valueArr = json_decode($valueObj, true);
        return $valueArr;
    }
}