Logic behind plugin system?

Programmer Question

I have an application in PHP (private CMS) that I would like to rewrite and add some new things - I would like to be able to extend my app in an easier way - through plugins

But the problem is - I don't know how to achieve "pluggability", how to make system that recognizes plugins and injects them into the app?

So, what's the logic of a simple plugin system?

Find the answer here