Used in conjunction with the JavaScript SDK, the PHP SDK can share user sessions seamlessly across the client and server. If a user is already logged in to their Facebook account, the JavaScript SDK will pick up the login session and send it to the server via a cookie, which the PHP SDK reads without any intervention on the developer's part
1. output a simple hardcoded error message
2. output a customized message
3. redirect to a local URL-path to handle the problem/error
4. redirect to an external URL to handle the problem/error