Unique ID for each registered user


New Member
My web app has been designed in PHP using mySQL as the database.
I have an option for users to register as a user.
One of the key items is we supply certain content based upon each users registration so we can't use their email address.
Ideally I would like to get the users IMEI number of their respective device, but this seems to not be available.
How through App Yet can I get each users unique device id (if not the imei then an id which is always the same for that device), and pass that through to my app so I can store it?