Best way to build a wizard with CodeIgniter?

[eluser]WanWizard[/eluser]
Keep track of the current step in the session.
Let's session says 2, and someone requests 4, just redirect back to 2. If nothing present in the session, redirect to 1.

Session data is very difficult to tamper with, especially if you encrypt it (which you should). If you already have a database active at this point, go for database sessions instead of cookie sessions. That way no user data will be send to the client, just a cookie with the encrypted session_id. The session library also takes care of session_id rotation (default every 300 seconds), so even if someone steals the cookie, they have to use it to hack in within 300 seconds, after which the stolen cookie is invalid.