微信换取openid的值

$appid = "";
$secret = "";
$js_code = I('code');
$grant_type = "authorization_code";
$openidurl="https://api.weixin.qq.com/sns/jscode2session?appid=".$appid."&secret=".$secret."&js_code=".$js_code."&grant_type=".$grant_type;
//print_r($openidurl);die;
$ch = curl_init();//初始化curl

curl_setopt($ch, CURLOPT_URL, $openidurl);//抓取指定网页

curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);//要求结果为字符串且输出到屏幕上

curl_setopt($ch, CURLOPT_HEADER,0);//设置header

curl_setopt($ch, CURLOPT_TIMEOUT,10);

$output = curl_exec($ch);

curl_close($ch);