Is there a way to save a "Client ID" trough PHP?

I have to build a (PHP based) landingspage that is used on WIFI access points (HP Aruba). The requirement would be: "If already filled the form for access, just show www.google.com (without loging in or something), if not, show them the form".

I would have done that by registering their MAC adresses, but I found out that was impossible trough PHP.

Do you guys have any idea of how I could achieve this?

Thank you already !