Json Question

I have an interface to an API that returns a json object.

The normal response would be something like

{  "response": { "status":"OK","id":241  }}

I decode the object as follows and check to see if it is ok.

$obj = json_decode($jsonObject);
$ok = $obj->response->status;

The problem exists if there is an error.
The JSON then looks like

{ "response": { "error_id":"NOAUTH",....}}

and I would get it's error code 

$error_id = $obj->response->error_id;

If I code it to check the error it will blow up with an error from Yii as follows:

Undefined property: stdClass::$error_id

Because the error does not exist.

How best to handle this ?

Is there a Yii function to check it?

Something like isset( $obj->response->error_id ) ?

This problem is killing me, I can't handle exceptions very gracefully!

Thanks for any advice!