Nick Halsted has posted the first two in a series of programming tips, not necessarily specific to PHP, but still very useful.

Over the years I have built up what I call my 'programming wisdom' this is what has replaced my early years of supposed 'raw talent'. [...] So I thought to myself what have I learnt that I can put across that will be useful to my fellow programmers who have not made all the mistakes I have. I plan on writing a long series of programming tips over the coming weeks (disturbed only by solicitors, investors and other scary programming time eating monsters).

The first two of the series are:

• Don't Repeat Yourself - make reusable code to save you time in the long run.
• Test, test, test - finding bugs in software before it's too late.