The web is full of useful advice focussed on pushing out the last bit of performance of your code. They mention trivial changes. like changing every occurrence of print with echo even suggesting to use for instead of foreach. These optimisations help, but you are not going to notice it unless they're in a tight loop with many iterations. It is also a wrong approach for tackling performance issues. Before you can optimise, you need to find out if your codeis actually slow; then you need to *understand* the code; and *then* you need to find out where you can optimise it. This talk introduces tools and concepts to optimise the optimisation of your PHP applications.
You can find Derick's talk slides over on his site [PDF]