Stefan Mischook has a (very) basic introduction video posted showing how to call functions/methods from another class outside the one you're currently using.

Recently I was asked by someone how they could call a function found in one class, in another. This may seem like basic stuff to those of us who know...but please keep in mind, at one time, none of us knew anything!

You can check out the video here. Be sure to check out more of the great videos he's created too on topics ranging from MVC frameworks to a beginner's guide to PHP.