In PHP : How to detect that DropBox won't hang?

Hi

I'm following the example PHP for authentication - no problem.

1. you create a link for them to go to DropBox page
2. They "allow" and are given a authorization code
3. they paste that into your awaiting code
4. you can the call : list($accessToken, $dropboxUserId) = $webAuth->finish($authCode); to obtain $accessToken.

BUT if the user DOES NOT authenticate and obtain a valid authorisation code from the dropbox page -
and they paste junk into your code AND you then call the above list(....

DropBox does not return at all and a blank screen is displayed.

I've tried everything to detect that it's OK to make the call - but can't find anything that would indicate $authCode is not genuine and a crash will follow.

Any ideas....???

Thanks