微信小程序支付流程：

• 小程序登录，获取openid
• 后端调用微信支付统一下单
• 得到预支付交易会话标识prepay_id
• 组装小程序支付所需的几个参数，并进行签名
• 支付成功，主动查询订单支付状态
• 接收微信支付回调通知

微信小程序支付：

<?php
/**
 * 微信小程序支付
 * @author Stephen
 */
namespace App\Helper\Pay;

use GuzzleHttp\Client;

class WxApp
{
    private $app_id;

    private $secret;

    private $mch_id;

    private $wx_pay_url = 'https://api.mch.weixin.qq.com';

    private $pay_uri = '/pay/unifiedorder';

    private $order_query_uri = '/pay/orderquery';

    public function __construct()
    {
        $this->app_id = 'app_id';
        $this->secret = 'secret';
        $this->mch_id = 'mch_id';
    }

    /**
     * 下单
     * @param float $totalFee
     * @param string $orderNo
     * @param string $orderName
     * @param string $notifyUrl
     * @param string $openId
     * @return array
     * @throws \GuzzleHttp\Exception\GuzzleException|\Exception
     */
    public function pay(float $totalFee, string $orderNo, string $orderName, string $notifyUrl, string $openId) : array
    {
        $data = [
            'appid' => $this->app_id,
            'mch_id' => $this->mch_id,
            'nonce_str' => $this->generateNonceStr(),
            'sign_type' => 'MD5',
            'body' => $orderName,
            'out_trade_no' => $orderNo,
            'fee_type' => 'CNY',
            'total_fee' => intval($totalFee * 100), // 单位：分
            'spbill_create_ip' => request()->getClientIp(),
            'time_start' => date('YmdHis'),
            'notify_url' => $notifyUrl,
            'trade_type' => 'JSAPI',
            'openid' => $openId
        ];
        $data['sign'] = $this->generateSignature($data);
        $payXmlStr = $this->arrayToXml($data);

        $unifiedOrder = $this->request($this->pay_uri, $payXmlStr);
        if (!$unifiedOrder) {
            throw new \Exception('支付请求失败');
        }
        if ($unifiedOrder['return_code'] != 'SUCCESS') {
            throw new \Exception($unifiedOrder['return_msg'] ?? '支付请求失败');
        }
        if ($unifiedOrder['result_code'] != 'SUCCESS') {
            throw new \Exception($unifiedOrder['err_code'] ?? '支付请求失败');
        }
        return $this->getWxPayData($unifiedOrder);
    }

    /**
     * 回调
     * @return string
     * @throws \Exception
     */
    public function notify($notify)
    {
        if (!$notify) {
            throw new \Exception('Parse XML Error');
        }
        if ($notify['return_code'] != 'SUCCESS') {
            throw new \Exception($notify['return_msg']));
        }
        if ($notify['result_code'] != 'SUCCESS') {
            throw new \Exception($notify['err_code']));
        }
        //签名验证
        $signature = $notify['sign'];
        $array = (array)$notify;
        unset($array['sign']);
        if ($signature != $this->generateSignature($array)) {
            throw new \Exception('签名验证失败');
        }
        return '<xml><return_code><![CDATA[SUCCESS]]></return_code><return_msg><![CDATA[OK]]></return_msg></xml>';
    }

    /**
     * 查询订单
     * @param string $orderNo
     * @param string $wxOrderNo
     * @return \SimpleXMLElement
     * @throws \GuzzleHttp\Exception\GuzzleException
     */
    public function query(string $orderNo, ?string $wxOrderNo='')
    {
        $data = [
            'appid' => $this->app_id,
            'mch_id' => $this->mch_id,
            'out_trade_no' => $orderNo,
            'nonce_str' => $this->generateNonceStr(),
            'sign_type' => 'MD5'
        ];
        $wxOrderNo && $data['transaction_id'] = $wxOrderNo;
        $data['sign'] = $this->generateSignature($data);
        $payXmlStr = $this->arrayToXml($data);

        $unifiedOrder = $this->request($this->order_query_uri, $payXmlStr);
        if (!$unifiedOrder) {
            throw new \Exception('订单查询失败');
        }
        if ($unifiedOrder['return_code'] != 'SUCCESS') {
            throw new \Exception($unifiedOrder['return_msg'] ?? '订单查询失败');
        }
        if ($unifiedOrder['result_code'] != 'SUCCESS') {
            throw new \Exception($unifiedOrder['err_code'] ?? '订单查询失败');
        }
        return $unifiedOrder;
    }

    /**
     * 生成随机字符串
     * @param int|null $length
     * @return string
     */
    private function generateNonceStr(?int $length = 16)
    {
        $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
        $str   = '';
        for ($i = 0; $i < $length; $i++) {
            $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
        }
        return $str;
    }

    /**
     * 生成签名
     * @param array $params
     * @return string
     */
    private function generateSignature(array $params)
    {
        ksort($params, SORT_STRING);
        $formatStr = $this->formatQueryStr($params);
        return strtoupper(md5($formatStr));
    }

    private function formatQueryStr(array $params)
    {
        $formatStr = "";
        foreach ($params as $key => $val) {
            $formatStr .= "{$key}={$val}&";
        }

        return $formatStr."key=".$this->secret;
    }

    private function arrayToXml(array $params)
    {
        $xml = "<xml>";
        foreach ($params as $key => $val) {
            if (is_numeric($val)) {
                $xml .= "<" . $key . ">" . $val . "</" . $key . ">";
            } else
                $xml .= "<" . $key . "><![CDATA[" . $val . "]]></" . $key . ">";
        }
        $xml .= "</xml>";
        return $xml;
    }

    /**
     * 微信小程序调用微信支付的参数
     * @param $unifiedOrder
     * @return array
     */
    private function getWxPayData($unifiedOrder) : array
    {
        $data = [
            'appId' => $this->app_id,
            'timeStamp' => time(),
            'nonceStr' => $this->generateNonceStr(),
            'package' => "prepay_id=" . $unifiedOrder['prepay_id'],
            'signType' => 'MD5'
        ];
        $data['paySign'] = $this->generateSignature($data);
        return $data;
    }

    /**
     * 微信支付请求
     * @param string $url
     * @param string|null $method
     * @param string|null $body
     * @return null|\SimpleXMLElement
     * @throws \GuzzleHttp\Exception\GuzzleException
     */
    private function request(string $url, string $params)
    {
        $url = $this->wx_pay_url . $url;
        $client = new Client(['verify' => false]);
        $response = $client->request('POST', $url, [
            'body' => $body
        ]);
        $code = $response->getStatusCode();
        if ($code === 200) {
            $content = $response->getBody()->getContents();
            return simplexml_load_string($content, 'SimpleXMLElement', LIBXML_NOCDATA);
        }
        return null;
    }
}

发起支付请求

$totalPrice = 100; //总金额，单位：元
$orderNo = date('Ymd') . rand(10000, 99999); // 订单号
$proName = '产品名称';
$callback = 'https://xx.com/pay/wxapp/callback'; // 支付成功后回调Url
$openId = ''; // OPEN ID

$wxAppPay = new App\Helper\Pay\WxApp();
// 将支付请求参数直接返回给前端，前端再到微信发起支付请求
return $wxAppPay->pay($totalPrice, $orderNo, $proName, $callback, $openId);

支付完成，主动查询订单支付状态

$orderNo = $_POST['order_no'];

$wxAppPay = new App\Helper\Pay\WxApp();
$response = $wxAppPay->query($orderNo);

if ($response['trade_state'] === 'SUCCESS') {
    // todo 支付成功，修改订单状态
}

支付回调

$post = file_get_contents('php://input');
$notify      = simplexml_load_string($post, 'SimpleXMLElement', LIBXML_NOCDATA);
$totalFee    = $notify['total_fee']; // 总金额
$cashFee     = $notify['cash_fee']; // 支付金额
$outOrderId  = $notify['transaction_id']; // 微信订单号
$orderId     = $notify['out_trade_no']; // 订单号
$paymentTime = $notify['time_end']; // 完成支付时间

// 回调验签
$wxAppPay = new App\Helper\Pay\WxApp();
$response = $wxAppPay->notify($notify);

// todo 完成支付，处理订单