How to implement a plugin system?

Hello all, I am working on a project which needs to have some sort of plugins architecture and I am kinda stuck. Basically I want to give a list of items to the user, and according to his/her selection, I want to load relevant functionality into my application. I was thinking of having an abstract plugin class, and have the plugins implement that but then how would I actually load the plugins? Say for instance I want to load plugins X,Y,Z (and lets say i implemented them as [X|Y|Z].class.php) , should I just 'include' (or require) them? Or should I initialize all possible plugins and just pick the ones user has chosen (which sounds a bit pointless as it would load unnecessary stuff)? How would you go about doing something like this? Thanks.