Pushing variable from one file to another...

[eluser]rockstyle[/eluser]
How to push some variable from one function to other inside one class. From now i'm using form_hidden but it's really dangerous, because when you edit some user data, you can easily change the value of this form_hidden and change some data of other user.
Can I use variables which was declared one page before, and make it be available for second page? (it's for my forms) I'm looking for option that it'll not be visible in html, not be visible in url bar as some uri segment and not to push it to some cookie, session data...

Any help appreciated.