One of the more challenging things I've run into while doing PHP development is effectively separating presentation from program logic. In many projects, I felt I was doing a great job until I had to do a markup change and jump through a lot hurdles to make it happen. In my recent projects, I've starting using the HTML Template IT extension in PEAR, and its made things a lot easier.

By way of example, he gives code blocks - one showing an example template and the other how to populate it with your own data. You can get more information on the extension from it's PEAR page.