How to get openid on WeChat

function test(){

        $code=$_GET['code'];

        //echo htmlspecialchars($code);exit;

        $appid='wx**********';  //APPID

        $appsecret='**************';  //APP密钥

        $url='https://api.weixin.qq.com/sns/oauth2/access_token?appid='.$appid.'&secret='.$appsecret.'&code='.$code.'&grant_type=authorization_code';

        $header [] = "content-type: application/json; charset=UTF-8";

        $ch = curl_init ();

        curl_setopt ( $ch, CURLOPT_URL, $url );

        //请求的方式是post

        curl_setopt ( $ch, CURLOPT_CUSTOMREQUEST, "GET" );

        // 禁用后cURL将终止从服务端进行验证

        curl_setopt ( $ch, CURLOPT_SSL_VERIFYPEER, FALSE );

        //不检查证书

        curl_setopt ( $ch, CURLOPT_SSL_VERIFYHOST, FALSE );

        //发送头部字段

        curl_setopt ( $ch, CURLOPT_HTTPHEADER, $header );

        //告诉对方 自己的浏览器型号

        curl_setopt ( $ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)' );

        //即表示自动进行跳转抓取（如果URL发生了302重定向）继续抓取

        curl_setopt ( $ch, CURLOPT_FOLLOWLOCATION, 1 );

        //自动重定向开启

        curl_setopt ( $ch, CURLOPT_AUTOREFERER, 1 );

        //发送的post参数

        //curl_setopt ( $ch, CURLOPT_POSTFIELDS, $param );

        //得到的结果不显示在屏幕上，作为变量结果储存

        curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );

        $res = curl_exec ( $ch );

        //如果运行错误，返回一个错误号

        $flat = curl_errno ( $ch );

        if ($flat) {

            $data = curl_error ( $ch );

        }

        curl_close ( $ch );

        //拿到了返回结果后json格式转化为可以使用的数组格式

        $res = json_decode ( $res, true );

        //$access_token=$res['access_token'];

        $openid=$res['openid'];

        //跳转到抽奖页面

        header('Location: http://wx.xxoo.com/test/index/index/openid/'.$openid);

    }