kamakasi:
Not exactly sure what you mean, but I would assume that when someone registers you are storing their name, password, etc. in a database.
If you had a field in your database such as regnum (for registration number) and set it to auto-increment, then each time someone registered the registration number would be increase by one. You could then write a simple query that would display the registration number, name, etc..
One problem with this would be that if you delete someone from the database the registration number would also be deleted and so the numbers would no longer be sequential.
You could also write the names of registered people into an array.
I am sure there are many, many ways you could do this, but without more information it is kind of difficult to give you an exact answer.
