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);
    }