How to use a function from a other controller?

[eluser]Colin Williams[/eluser]
If you find yourself needing to call another controller's function, you've probably not built your controllers correctly. The function that needs calling typically is better suited as a model function. When you are checking credentials, you are essentially asking either, "What is the current state of the user?" or "Do these credentials match a user's?" Models represent the "state" of your application, i.e. resource data.

Short answer is: Put it in the model where it belongs.