PHP获取小程序用户openid

代码如下：

//code值，从前端获取
$code = $_GET['code'];

//注册时获取
$app;
$secret = "xxxxxxxxxxxxxxxxxxxxx";

$get_code_url = 'https://api.weixin.qq.com/sns/jscode2session?app&secret='.$secret.'&js_code='.$code.'&grant_type=authorization_code';
$ret=file_get_contents($get_code_url);
$openopenid'};