开发微信支付需要认证服务号并且已经开通微信支付，获得微信支付商ID，设置apikey了的。

官方给了公众号支付的php源码SDK
但是这个SDK文件非常多，源码乱七八糟的，真的一开始接触的人拿着官方的SDK肯定踩坑。
而且还不知道最后能不能用。

我看到了一个作者写的源码，还不错，我简单进行优化了，只有200多行代码就可以实现公众号支付。

在开发之前，得先配置网页授权的域名和支付授权域名。网页授权域名在公众号后台，接口权限找到网页获取用户基本信息处配置你的代码所在的目录，例如你的公众号支付代码放在www.baidu.com/pay/jspai.php

那么网页授权域名为：www.baidu.com/pay/

支付授权域名配置要登录商户平台

然后往下拉找到公众号支付

添加授权目录即可，授权目录也是你的代码所在目录。

接着就可以开发了下面是代码，直接可以用。

<?php
header('Content-type:text/html; Charset=utf-8');
$mchid = '微信支付商户号';
$appid = '微信支付申请对应的公众号的APPID';
$appKey = '微信支付申请对应的公众号的APPSECRET'; 
$apiKey = 'API密钥';   //商户平台-帐户设置-安全设置-API安全-API密钥-设置API密钥
//①、获取用户openid
$wxPay = new WxpayService($mchid,$appid,$appKey,$apiKey);
$openId = $wxPay->GetOpenid();      //获取openid
if(!$openId) exit('获取openid失败');
//②、统一下单
$outTradeNo = uniqid();     //你自己的商品订单号
$payAmount = 0.01;          //付款金额，单位:元
$orderName = 'test';    //订单标题
$notifyUrl = 'https://www.likeyunba.com/pay/notify.php';     //付款成功后的回调地址(不要有问号)
$payTime = time();      //付款时间
$jsApiParameters = $wxPay->createJsBizPackage($openId,$payAmount,$outTradeNo,$orderName,$notifyUrl,$payTime);
$jsApiParameters = json_encode($jsApiParameters);
?>
    <html>
    <head>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1"/>
        <title>微信支付样例-支付</title>
        <script type="text/javascript">
            //调用微信JS api 支付
            function jsApiCall()
            {
                WeixinJSBridge.invoke(
                    'getBrandWCPayRequest',
                    <?php echo $jsApiParameters; ?>,
                    function(res){
                        WeixinJSBridge.log(res.err_msg);
                        //alert(res.err_code+res.err_desc+res.err_msg);
                        if(res.err_msg == "get_brand_wcpay_request:ok"){
                            //支付成功跳转页面
                            window.location.href="http://www.likeyunba.com/pay/true.html";
                        }else{
                            //支付失败/或取消支付跳转页面
                            window.location.href="http://www.likeyunba.com/pay/false.html";
                        }
                    }
                );
            }
            function callpay()
            {
                if (typeof WeixinJSBridge == "undefined"){
                    if( document.addEventListener ){
                        document.addEventListener('WeixinJSBridgeReady', jsApiCall, false);
                    }else if (document.attachEvent){
                        document.attachEvent('WeixinJSBridgeReady', jsApiCall);
                        document.attachEvent('onWeixinJSBridgeReady', jsApiCall);
                    }
                }else{
                    jsApiCall();
                }
            }
        </script>
    </head>
    <body>
    <br/>
    <font color="#9ACD32"><b>该笔订单支付金额为<span style="color:#f00;font-size:50px"><?php echo $payAmount?>元</span>钱</b></font><br/><br/>
    <div align="center">
        <button style="width:210px; height:50px; border-radius: 15px;background-color:#FE6714; border:0px #FE6714 solid; cursor: pointer;  color:white;  font-size:16px;" type="button" onclick="callpay()" >立即支付</button>
    </div>
    </body>
    </html>
<?php
header("Content-Type:text/html; charset=utf-8");
class WxpayService
{
    protected $mchid;
    protected $appid;
    protected $appKey;
    protected $apiKey;
    public $data = null;
    public function __construct($mchid, $appid, $appKey,$key)
    {
        $this->mchid = $mchid; //https://pay.weixin.qq.com 产品中心-开发配置-商户号
        $this->appid = $appid; //微信支付申请对应的公众号的APPID
        $this->appKey = $appKey; //微信支付申请对应的公众号的APP Key
        $this->apiKey = $key;   //https://pay.weixin.qq.com 帐户设置-安全设置-API安全-API密钥-设置API密钥
    }
    /**
     * 通过跳转获取用户的openid，跳转流程如下：
     * 1、设置自己需要调回的url及其其他参数，跳转到微信服务器https://open.weixin.qq.com/connect/oauth2/authorize
     * 2、微信服务处理完成之后会跳转回用户redirect_uri地址，此时会带上一些参数，如：code
     * @return 用户的openid
     */
    public function GetOpenid()
    {
        //通过code获得openid
        if (!isset($_GET['code'])){
            //触发微信返回code码
            $scheme = $_SERVER['HTTPS']=='on' ? 'https://' : 'http://';
            $baseUrl = urlencode($scheme.$_SERVER['HTTP_HOST'].$_SERVER['PHP_SELF'].$_SERVER['QUERY_STRING']);
            $url = $this->__CreateOauthUrlForCode($baseUrl);
            Header("Location: $url");
            exit();
        } else {
            //获取code码，以获取openid
            $code = $_GET['code'];
            $openid = $this->getOpenidFromMp($code);
            return $openid;
        }
    }
    /**
     * 通过code从工作平台获取openid机器access_token
     * @param string $code 微信跳转回来带上的code
     * @return openid
     */
    public function GetOpenidFromMp($code)
    {
        $url = $this->__CreateOauthUrlForOpenid($code);
        $res = self::curlGet($url);
        //取出openid
        $data = json_decode($res,true);
        $this->data = $data;
        $openid = $data['openid'];
        return $openid;
    }
    /**
     * 构造获取open和access_toke的url地址
     * @param string $code，微信跳转带回的code
     * @return 请求的url
     */
    private function __CreateOauthUrlForOpenid($code)
    {
        $urlObj["appid"] = $this->appid;
        $urlObj["secret"] = $this->appKey;
        $urlObj["code"] = $code;
        $urlObj["grant_type"] = "authorization_code";
        $bizString = $this->ToUrlParams($urlObj);
        return "https://api.weixin.qq.com/sns/oauth2/access_token?".$bizString;
    }
    /**
     * 构造获取code的url连接
     * @param string $redirectUrl 微信服务器回跳的url，需要url编码
     * @return 返回构造好的url
     */
    private function __CreateOauthUrlForCode($redirectUrl)
    {
        $urlObj["appid"] = $this->appid;
        $urlObj["redirect_uri"] = "$redirectUrl";
        $urlObj["response_type"] = "code";
        $urlObj["scope"] = "snsapi_base";
        $urlObj["state"] = "STATE"."#wechat_redirect";
        $bizString = $this->ToUrlParams($urlObj);
        return "https://open.weixin.qq.com/connect/oauth2/authorize?".$bizString;
    }
    /**
     * 拼接签名字符串
     * @param array $urlObj
     * @return 返回已经拼接好的字符串
     */
    private function ToUrlParams($urlObj)
    {
        $buff = "";
        foreach ($urlObj as $k => $v)
        {
            if($k != "sign") $buff .= $k . "=" . $v . "&";
        }
        $buff = trim($buff, "&");
        return $buff;
    }
    /**
     * 统一下单
     * @param string $openid 调用【网页授权获取用户信息】接口获取到用户在该公众号下的Openid
     * @param float $totalFee 收款总费用 单位元
     * @param string $outTradeNo 唯一的订单号
     * @param string $orderName 订单名称
     * @param string $notifyUrl 支付结果通知url 不要有问号
     * @param string $timestamp 支付时间
     * @return string
     */
    public function createJsBizPackage($openid, $totalFee, $outTradeNo, $orderName, $notifyUrl, $timestamp)
    {
        $config = array(
            'mch_id' => $this->mchid,
            'appid' => $this->appid,
            'key' => $this->apiKey,
        );
        $orderName = iconv('GBK','UTF-8',$orderName);
        $unified = array(
            'appid' => $config['appid'],
            'attach' => 'pay',             //商家数据包，原样返回，如果填写中文，请注意转换为utf-8
            'body' => $orderName,
            'mch_id' => $config['mch_id'],
            'nonce_str' => self::createNonceStr(),
            'notify_url' => $notifyUrl,
            'openid' => $openid,            //rade_type=JSAPI，此参数必传
            'out_trade_no' => $outTradeNo,
            'spbill_create_ip' => '127.0.0.1',
            'total_fee' => intval($totalFee * 100),       //单位 转为分
            'trade_type' => 'JSAPI',
        );
        $unified['sign'] = self::getSign($unified, $config['key']);
        $responseXml = self::curlPost('https://api.mch.weixin.qq.com/pay/unifiedorder', self::arrayToXml($unified));
        $unifiedOrder = simplexml_load_string($responseXml, 'SimpleXMLElement', LIBXML_NOCDATA);
        if ($unifiedOrder === false) {
            die('parse xml error');
        }
        if ($unifiedOrder->return_code != 'SUCCESS') {
            die($unifiedOrder->return_msg);
        }
        if ($unifiedOrder->result_code != 'SUCCESS') {
            die($unifiedOrder->err_code);
        }
        $arr = array(
            "appId" => $config['appid'],
            "timeStamp" => "$timestamp",        //这里是字符串的时间戳，不是int，所以需加引号
            "nonceStr" => self::createNonceStr(),
            "package" => "prepay_id=" . $unifiedOrder->prepay_id,
            "signType" => 'MD5',
        );
        $arr['paySign'] = self::getSign($arr, $config['key']);
        return $arr;
    }
    public static function curlGet($url = '', $options = array())
    {
        $ch = curl_init($url);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_TIMEOUT, 30);
        if (!empty($options)) {
            curl_setopt_array($ch, $options);
        }
        //https请求 不验证证书和host
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
        $data = curl_exec($ch);
        curl_close($ch);
        return $data;
    }
    public static function curlPost($url = '', $postData = '', $options = array())
    {
        if (is_array($postData)) {
            $postData = http_build_query($postData);
        }
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $postData);
        curl_setopt($ch, CURLOPT_TIMEOUT, 30); //设置cURL允许执行的最长秒数
        if (!empty($options)) {
            curl_setopt_array($ch, $options);
        }
        //https请求 不验证证书和host
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
        $data = curl_exec($ch);
        curl_close($ch);
        return $data;
    }
    public static function createNonceStr($length = 16)
    {
        $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
        $str = '';
        for ($i = 0; $i < $length; $i++) {
            $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
        }
        return $str;
    }
    public static function arrayToXml($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 $xml;
    }
    public static function getSign($params, $key)
    {
        ksort($params, SORT_STRING);
        $unSignParaString = self::formatQueryParaMap($params, false);
        $signStr = strtoupper(md5($unSignParaString . "&key=" . $key));
        return $signStr;
    }
    protected static function formatQueryParaMap($paraMap, $urlEncode = false)
    {
        $buff = "";
        ksort($paraMap);
        foreach ($paraMap as $k => $v) {
            if (null != $v && "null" != $v) {
                if ($urlEncode) {
                    $v = urlencode($v);
                }
                $buff .= $k . "=" . $v . "&";
            }
        }
        $reqPar = '';
        if (strlen($buff) > 0) {
            $reqPar = substr($buff, 0, strlen($buff) - 1);
        }
        return $reqPar;
    }
}
?>

notify.php

<?php
/**
 * 原生支付（扫码支付）及公众号支付的异步回调通知
 * 说明：需要在native.php或者jsapi.php中的填写回调地址。例如：http://www.xxx.com/wx/notify.php
 * 付款成功后，微信服务器会将付款结果通知到该页面
 */
header('Content-type:text/html; Charset=utf-8');
$mchid = '微信支付商户号';
$appid = '公众号APPID'; 
$apiKey = 'API密钥'; 
$wxPay = new WxpayService($mchid,$appid,$apiKey);
$result = $wxPay->notify();
if($result){
    //完成你的逻辑
    //例如连接数据库，获取付款金额$result['cash_fee']，获取订单号$result['out_trade_no']，修改数据库中的订单状态等;
}else{
    echo 'pay error';
}
class WxpayService
{
    protected $mchid;
    protected $appid;
    protected $apiKey;
    public function __construct($mchid, $appid, $key)
    {
        $this->mchid = $mchid;
        $this->appid = $appid;
        $this->apiKey = $key;
    }
    public function notify()
    {
        $config = array(
            'mch_id' => $this->mchid,
            'appid' => $this->appid,
            'key' => $this->apiKey,
        );
        $postStr = $GLOBALS["HTTP_RAW_POST_DATA"];
        $postObj = simplexml_load_string($postStr, 'SimpleXMLElement', LIBXML_NOCDATA);
        if ($postObj === false) {
            die('parse xml error');
        }
        if ($postObj->return_code != 'SUCCESS') {
            die($postObj->return_msg);
        }
        if ($postObj->result_code != 'SUCCESS') {
            die($postObj->err_code);
        }
        $arr = (array)$postObj;
        unset($arr['sign']);
        if (self::getSign($arr, $config['key']) == $postObj->sign) {
            echo '<xml><return_code><![CDATA[SUCCESS]]></return_code><return_msg><![CDATA[OK]]></return_msg></xml>';
            return $arr;
        }
    }
    /**
     * 获取签名
     */
    public static function getSign($params, $key)
    {
        ksort($params, SORT_STRING);
        $unSignParaString = self::formatQueryParaMap($params, false);
        $signStr = strtoupper(md5($unSignParaString . "&key=" . $key));
        return $signStr;
    }
    protected static function formatQueryParaMap($paraMap, $urlEncode = false)
    {
        $buff = "";
        ksort($paraMap);
        foreach ($paraMap as $k => $v) {
            if (null != $v && "null" != $v) {
                if ($urlEncode) {
                    $v = urlencode($v);
                }
                $buff .= $k . "=" . $v . "&";
            }
        }
        $reqPar = '';
        if (strlen($buff) > 0) {
            $reqPar = substr($buff, 0, strlen($buff) - 1);
        }
        return $reqPar;
    }
}

简单说明：
1】新建jsapi.php，拷贝上面的代码，修改商户号、appid、appsecret、密钥即可，还有下面的回调地址，前面的域名需要换成你的nofity.php的地址。至于支付成功和支付失败跳转的页面按自己的需求修改即可。
要注意，订单标题不能为中文，否则会提示body参数错误什么的。
2】新建nofity.php
把上面的代码拷贝上去，修改商户号、appid、密钥即可。
上传到支付授权目录，然后访问jsapi.php页面测试。