调用支付功能的前提是得去公众平台申请认证权限:https://pay.weixin.qq.com/wiki/doc/api/wxa/wxa_api.php?chapter=7_10&index=1
官方业务流程:

主要是把订单号、openid、金额等信息传递到后台服务器,后台服务器,然后根据官方文档进行一些数据格式转换,最后调用统一下单接口:https://api.mch.weixin.qq.com/pay/unifiedorder,实际开发中有遇到的问题主要就是签名的生成不对.
小程序代码:

/*** 微信支付接口*/wxPaymoney:function (out_trade_no, total_fee, openid){    //out_trade_no 后台统一下单接口需要用var that = thiswx.request({method: 'POST',data: {openid: openid,    //调用人的openidout_trade_no: out_trade_no,body: '答题奖金',  notify_url: 'https://www.q**************.php',//后台接口自动修改订单状态total_fee: total_fee//支付金额},url: '',        //服务器接口（微信统一下单接口）header: {'Content-Type': 'application/x-www-form-urlencoded'},success: function (res) {console.log('调起支付')wx.requestPayment({                //成功之后，调用小程序微信支付'timeStamp': res.data.timeStamp,'nonceStr': res.data.nonceStr,'package': res.data.package,'signType': 'MD5','paySign': res.data.paySign,success: function (res) {console.log(res)wx.showToast({title: '支付成功',icon: 'success',duration: 3000})}, fail: function (res) {console.log('付款失败');wx.showModal({title: '提醒',content: '付款失败,如要出题请重新编辑题目',showCancel: false,success:function(){    }})return},})},fail: function (res) {console.log(res.data)}})}

PHP后台
pay.php

<?php
include_once "WeixinPay.php";
$appid = '';  
$mch_id='';  
$key='';
$openid = trim($_POST['openid']);
//$out_trade_no = $mch_id.time();
$out_trade_no = trim($_POST['out_trade_no']);
//$body = "答题奖金";
$body = $_POST['body'];
//$notify_url = '';
$notify_url = trim($_POST['notify_url']);
$total_fee = floatval($_POST['total_fee']);
//根据需求加日志$weixinpay = new WeixinPay($appid,$openid,$mch_id,$key,$out_trade_no,$body,$total_fee,$notify_url);
$return=$weixinpay->pay();
echo json_encode($return);

WeixinPay.php

<?php/** 小程序微信支付*/
class WeixinPay {protected $appid;protected $mch_id;protected $key;protected $openid;protected $out_trade_no;protected $body;protected $total_fee;function __construct($appid, $openid, $mch_id, $key,$out_trade_no,$body,$total_fee,$notify_url) {$this->appid = $appid;$this->openid = $openid;$this->mch_id = $mch_id;$this->key = $key;$this->out_trade_no = $out_trade_no;$this->body = $body;$this->total_fee = $total_fee;$this->notify_url = $notify_url;}public function pay() {//统一下单接口$return = $this->weixinapp();return $return;}//微信小程序接口private function weixinapp() {//统一下单接口$unifiedorder = $this->unifiedorder();$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,'out_trade_no'=> $this->out_trade_no,'total_fee' => $this->total_fee,'spbill_create_ip' => '192.168.0.1', //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);$parameters['dd'] = ['sd'=>'sdsd','asda'=>'sdsd'];$xmlData = $this->arrayToXml($parameters);print_r($xmlData);die;$return = $this->xmlToArray($this->postXmlCurl($xmlData, $url, 60));return $return;}private static function postXmlCurl($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($ch, CURLOPT_RETURNTRANSFER, TRUE);//post提交方式curl_setopt($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);throw new WxPayException("curl出错，错误码:$error");}}//数组转换成xmlprivate function arrayToXml($arr) {$xml = "<root>";foreach ($arr as $key => $val) {if (is_array($val)) {$xml .= "<" . $key . ">" . $this->arrayToXml($val) . "</" . $key . ">";} else {$xml .= "<" . $key . ">" . $val . "</" . $key . ">";}}$xml .= "</root>";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] = $v;}// print_r($Parameters);//签名步骤一：按字典序排序参数ksort($Parameters);//print_r($Parameters);$String = $this->formatBizQueryParaMap($Parameters, false);//print_r($String);//签名步骤二：在string后加入KEY$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 . "=" . $v . "&";}//print_r($buff);$reqPar = '';if (strlen($buff) > 0) {$reqPar = substr($buff, 0, strlen($buff) - 1);}return $reqPar;}
}
?>