Accessing the Facebook signed_request parameter in iframe tab page

i was wondering how to access the Facebook signed_request parameter inside an iframe tab?

1. $_REQUEST['signed_request']
2. $_POST['signed_request']

Do I always ge开发者_如何学运维t the signed_request? Or do I need to authorize the user first?

Thanks,

---

Ohh the Facebook PHP SDK has a method for that (using $_REQUEST): $facebook->getSignedRequest()