Okay, maybe I exagerate a bit, but if you're developing PHP that others need to decipher, commenting your code is a must. Heck, I'm finding that code documentation for myself is a saving grace when revisiting what I've written months, or even a week later.
I'm going into the last day of a week long XML course at work. The class has been great, despite being completely Microsoft-centric. Despite this, the instructor has done a good job of covering the topics and inspiring many possible projects in my head. Most of these projects are for the day job and will focus mainly on XSL transformations of training manuals. So, my challenge was to get an XSL parser that works with PHP. A buddy at work informed me that I'd need Sablotron to do XSL with PHP.