I don't know this widget offhand, but it does seem to be problematic and discussed here. Have you searched this forum for past issues?
How about the online help for variables?
On the PHP question, what error are you referring to? Others seem to have this working fine:
Text in the Help about this certificate widget is not correct, sorry. You do not have to define a user variable yourself as was the case for Captivate 4. The user will be asked to enter his name when getting to the certificate and this will be stored in the automatically created variable. Sorry about that confusion, tried to post a comment in the Help about it if my memory is correct,