统一下单接口：

<?php

namespace pay;

class WxPay
{
    protected $appid;
    protected $mch_id;
    protected $key;
    protected $openid;
    protected $out_trade_no;
    protected $body;
    protected $total_fee;

    function __construct($openid, $out_trade_no, $body, $total_fee,$notify_url)
    {
        $this->appid = '';//appid
        $this->openid = $openid;//用户openid
        $this->mch_id = '';//mch_id
        $this->key = '';//key
        $this->out_trade_no = $out_trade_no;//订单号
        $this->body = $body;//订单描述
        $this->total_fee = $total_fee;//金额（分）
        $this->notify_url = $notify_url;//异步回调地址
    }

    public function pay()
    {
        //统一下单接口
        $return = $this->weixinapp();
        return $return;
    }
    //微信小程序接口
    private function weixinapp()
    {
        //统一下单接口
        $unifiedorder = $this->unifiedorder();
        if(!$unifiedorder){
            return false;
        }
        $parameters = array(
            'appId' => $this->appid, //小程序ID
            'timeStamp' => '' . time() . '', //时间戳
            'nonceStr' => $this->createNoncestr(), //随机串
            'package' => 'prepay_id=' . $unifiedorder['prepay_id'], //数据包
            'signType' => 'MD5'//签名方式
        );
        //签名
        $parameters['paySign'] = $this->getSign($parameters);
        return $parameters;
    }

    //统一下单接口
    private function unifiedorder()
    {
        $url = 'https://api.mch.weixin.qq.com/pay/unifiedorder';
        $parameters = array(
            'appid' => $this->appid, //小程序ID
            'mch_id' => $this->mch_id, //商户号
            'nonce_str' => $this->createNoncestr(), //随机字符串
            'body' => $this->body,//商品描述
            'out_trade_no' => $this->out_trade_no,//商户订单号
            'total_fee' => $this->total_fee,//总金额 单位 分
            'spbill_create_ip' => $_SERVER['REMOTE_ADDR'], //终端IP
            'notify_url' => $this->notify_url, //通知地址 确保外网能正常访问
            'openid' => $this->openid, //用户id
            'trade_type' => 'JSAPI'//交易类型
        );
        //统一下单签名
        $parameters['sign'] = $this->getSign($parameters);
        $xmlData = $this->arrayToXml($parameters);
        $return = $this->xmlToArray($this->postXmlCurl($xmlData, $url, 60));
        if($return['return_code'] == 'SUCCESS' && $return['result_code'] == 'SUCCESS'){
            return $return;
        }else{
            return false;
        }
    }

    private static function postXmlCurl($xml, $url, $second = 30)
    {
        $ch = curl_init();
        //设置超时
        curl_setopt($ch, CURLOPT_TIMEOUT, $second);
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE); //严格校验
        //设置header
        curl_setopt($ch, CURLOPT_HEADER, FALSE);
        //要求结果为字符串且输出到屏幕上
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
        //post提交方式
        curl_setopt($ch, CURLOPT_POST, TRUE);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
        curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 20);
        curl_setopt($ch, CURLOPT_TIMEOUT, 40);
        set_time_limit(0);
        //运行curl
        $data = curl_exec($ch);
        //返回结果
        if ($data) {
            curl_close($ch);
            return $data;
        } else {
            $error = curl_errno($ch);
            curl_close($ch);
            return $error;
        }
    }

    //数组转换成xml
    private function arrayToXml($arr)
    {
        $xml = "<xml>";
        foreach ($arr as $key => $val) {
            if (is_array($val)) {
                $xml .= "<" . $key . ">" . $this->arrayToXml($val) . "</" . $key . ">";
            } else {
                $xml .= "<" . $key . ">" . $val . "</" . $key . ">";
            }
        }
        $xml .= "</xml>";
        return $xml;
    }

    //xml转换成数组
    private function xmlToArray($xml)
    {
        //禁止引用外部xml实体
        libxml_disable_entity_loader(true);
        $xmlstring = simplexml_load_string($xml, 'SimpleXMLElement', LIBXML_NOCDATA);
        $val = json_decode(json_encode($xmlstring), true);
        return $val;
    }


    //作用：产生随机字符串，不长于32位
    private function createNoncestr($length = 32)
    {
        $chars = "abcdefghijklmnopqrstuvwxyz0123456789";
        $str = "";
        for ($i = 0; $i < $length; $i++) {
            $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
        }
        return $str;
    }

    //作用：生成签名
    private function getSign($Obj)
    {
        foreach ($Obj as $k => $v) {
            $Parameters[$k] = $v;
        }
        //签名步骤一：按字典序排序参数
        ksort($Parameters);
        $String = $this->formatBizQueryParaMap($Parameters, false);
        //签名步骤二：在string后加入KEY
        $String = $String . "&key=" . $this->key;
        //签名步骤三：MD5加密
        $String = md5($String);
        //签名步骤四：所有字符转为大写
        $result_ = strtoupper($String);
        return $result_;
    }

    ///作用：格式化参数，签名过程需要使用
    private function formatBizQueryParaMap($paraMap, $urlencode)
    {
        $buff = "";
        ksort($paraMap);
        foreach ($paraMap as $k => $v) {
            if ($urlencode) {
                $v = urlencode($v);
            }
            $buff .= $k . "=" . $v . "&";
        }
        $reqPar = '';
        if (strlen($buff) > 0) {
            $reqPar = substr($buff, 0, strlen($buff) - 1);
        }
        return $reqPar;
    }
}

异步回调：

<?php

namespace pay;

class WxPayCallback
{
    /**
     * @return string
     */
    public function notify($xmlstr)
    {
        $xmlObj = simplexml_load_string($xmlstr, 'SimpleXMLElement', LIBXML_NOCDATA);
        $data = json_decode(json_encode($xmlObj),true);
        if ($this->checkSign($data)) {
            //校验通过
            return $data;
        }
        return false;
    }
    /**校验签名
     * @param array $arr
     * @return bool
     */
    private function checkSign(array $arr): bool
    {
        $sign = $arr['sign'];
        unset($arr['sign']);
        $key=sysconfig('ms','wx_key');//微信商户平台支付设置的key
        ksort($arr);//根据key升序排序
        $str=http_build_query($arr);//数组字典排序拼接字符串
        $stringSignTemp = $str.'&key='.$key;//拼接上key
        $signValue = md5($stringSignTemp);//md5处理
        $signValue = strtoupper($signValue);//转大写
        return $signValue == $sign;
    }
}