条件：申请微信支付商户号，拥有
商户号id及商户号密钥

一、微信接口

https://developers.weixin.qq.com/miniprogram/dev/api/open-api/payment/wx.requestPayment.html#wxrequestpaymentobject


这几个参数需要通过wx.request从后台得到
前台代码：

  wx.request({                     url: '后台地址',data: {//根据需要所传参数},method: 'POST',header: {'content-type': 'application/x-www-form-urlencoded'},success: function (response) {// console.log(response.data);// 发起支付  出现支付页面wx.requestPayment({'timeStamp': response.data.timeStamp,'nonceStr': response.data.nonceStr,'package': response.data.package,'signType': 'MD5','paySign': response.data.paySign,success: function (res) {wx.showToast({title: '支付成功'});//  跳转页面wx.switchTab({url: '../index'})},fail:function(res){console.log(res);}});}});

二、后台代码（PHP）

一大部分与公众号是一样的，可以看这个人的视频来学习
https://www.imooc.com/u/197650/courses?sort=publish
代码里的很多算法原理讲的很清楚
上代码！
（代码我也是学习的，原链接在这里
https://segmentfault.com/a/1190000014444847）

<?php
date_default_timezone_set("PRC");
class PayController{private $appid=""; //小程序appidprivate $appsecret= ""; //小程序的secretprivate $MCHID=""; //商户号idprivate $KEY=""; //商户号key/**   发起支付回调支付信息*   by:leoyi*   Date:2018-4-8*/public function payOrder($total,$r_openid,$r_body,$r_trade_no){$total_fee = $total; //支付金额
//$total_fee = 1;$openid = $r_openid; //用户的Openidif(empty($total_fee) || empty($openid)){ //一定要有用户Openid和支付金额die("缺少参数!");}$appid = $this->appid;$MCHID = $this->MCHID; //商户号$KEY =  $this->KEY; //商户key$url = "https://api.mch.weixin.qq.com/pay/unifiedorder";$data['appid'] = $appid;  //小程序appid$data['mch_id'] = $MCHID;	//商户号id$data['nonce_str'] = md5($MCHID.time()); //验证的支付$data['openid'] = $openid; //用户openid$data['body'] = $r_body; //微信支付对应的商家/公司主体名$data['out_trade_no'] = $r_trade_no; //订单号id,用于回调改订单状态$data['total_fee'] = $total_fee; //支付金额，单位为分！！$data['spbill_create_ip'] = '8.8.8.8'; //验证ip地址，这个不用改随意$data['notify_url'] = ""; //微信支付成功的回调路径，必须是外网可以访问的链接，必须在支付文件的同目录下//不能带参数    //这是个巨大的坑，到现在也没懂，所以回调采用了别的方法，下面介绍$data['trade_type'] = "JSAPI"; //小程序支付，所以是JSAPI// --------------------以下这一串都不用改--------------------------------ksort($data);$sign_str = $this->ToUrlParams($data);$sign_str = $sign_str."&key=".$KEY;$data['sign'] = strtoupper(md5($sign_str));$xml = $this->arrayToXml($data);$r = $this->postXmlCurl($xml,$url,true);$result = json_decode($this->xml_to_json($r));//var_dump($result);if($result->return_code == 'SUCCESS'){$stringtime = date("Y-m-d H:i:s",time());$sdata['timeStamp'] = (string)strtotime($stringtime);    //必须为字符串类型$sdata['appId'] = $appid;// $sdata['timeStamp'] = time();$sdata['nonceStr'] = md5(time().rand().rand().$openid);if(isset($result->prepay_id)) {$sdata['package'] = "prepay_id=" . $result->prepay_id;  //必须带前面的prepay_id}$sdata['signType'] = "MD5";ksort($sdata);$sign_str = $this->ToUrlParams($sdata);$sign_str = $sign_str."&key=".$KEY;$sdata['paySign'] = strtoupper(md5($sign_str));echo json_encode($sdata);}else{echo "失败";}
// -----------------------都不用改-----------------------------------------------//这些方法如果不懂在上面的视频链接中都能找到讲解，这与公众号是一模一样的public function postXmlCurl($xml, $url, $useCert = false, $second = 10){$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($ch,CURLOPT_SSL_VERIFYHOST,FALSE);//严格校验2
//设置headercurl_setopt($ch, CURLOPT_HEADER, FALSE);
//要求结果为字符串且输出到屏幕上curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);curl_setopt($ch, CURLOPT_POST, TRUE);curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
//运行curl$data = curl_exec($ch);
//返回结果if($data){curl_close($ch);return $data;} else {$error = curl_errno($ch);curl_close($ch);return $error;}}public function ToUrlParams($data){$buff = "";foreach ($data as $k => $v){if($k != "sign" && $v != "" && !is_array($v)){$buff .= $k . "=" . $v . "&";}}$buff = trim($buff, "&");return $buff;}public function arrayToXml($arr){$xml = "<xml>";foreach ($arr as $key=>$val){if (is_numeric($val)){$xml.="<".$key.">".$val."</".$key.">";}else{$xml.="<".$key."><![CDATA[".$val."]]></".$key.">";}}$xml.="</xml>";return $xml;}public function  xml_to_json($xmlstring) {return json_encode($this->xml_to_array($xmlstring),JSON_UNESCAPED_UNICODE);}public function post_url($post_data, $url){$ch = curl_init();
//设置超时curl_setopt($ch, CURLOPT_TIMEOUT, 10);curl_setopt($ch,CURLOPT_URL, $url);curl_setopt($ch,CURLOPT_SSL_VERIFYPEER,FALSE);curl_setopt($ch,CURLOPT_SSL_VERIFYHOST,FALSE);//严格校验2curl_setopt($ch, CURLOPT_HEADER, FALSE);curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);curl_setopt($ch, CURLOPT_POST, TRUE);curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);$data = curl_exec($ch);curl_close($ch);return $data;}public function xml_to_array($xml) {
//return ((array) simplexml_load_string($xmlstring));return simplexml_load_string($xml,'SimpleXMLElement',LIBXML_NOCDATA);//return json_decode(xml_to_json($xmlstring));}
}//整个类写完记得要实例化
$total_fee = $_POST['totalFee'];   //支付金额
$openid = $_POST['openid'];
$body = $_POST['body'];   //商家名
$trade_no = $_POST['tradeNo'];  //订单编号$demo = new PayController();
$demo->payOrder($total_fee,$openid,$body,$trade_no);

三、支付成功后修改订单状态

在支付的后端代码里是有回调路径的，但是这个…反正我没搞明白，如果我搞明白了我再来补这个坑。
所以采用了另外的方法：在wx.requestPayment成功后再对后端进行请求再更改状态

就更容易理解了，注意局部变量的使用，保证参数有效。