PHP - Execute a specific php code when clicking a link
===

I'm currently working on a website and I would like to be able to do the following:

- when clicking one of the links from the sideMenu the only thing I would like to change would be the content of my contentMain div and nothing else(page layout/design/etc)

Could anybody give me some general pointers on how I could achieve this in php?

Thank You in advance :D