在curl中获取指定数组

Question

i get some data with curl from web service and here is my curl code:

$uri="http://test.com";
$ch=curl_init($uri);
curl_setopt_array($ch,array(
CURLOPT_HTTPHEADER=>array('shopid:691','device:2'),
CURLOPT_RETURNTRANSFER=>true
));
$out=curl_exec($ch);
curl_close($ch);
$obj=json_decode($out,true);
print_r($obj);

and my curl out put:

{
  "p":
    "Serial": "b216"
  },
  "review": {
    "Review": "<div dir=\"rtl\">test</div>
"
  },
  "likes": 2,
  "brand": {
    "Name": "test"
  },
  "followers": [],
  "Shop": null
}

i want to get "b216" value in array "p" but i cannot get the value with print_r or echo $obj[0] or echo $obj[0]['Serial']

anyone know how to get this data?