The Ibuildings techPortal has posted the latest episode of their recordings from this year's Dutch PHP Conference - Derick Rethans' talk "Profiling PHP Applications".
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.
You can listen to this latest episode either through the in-page player, by downloading the mp3 or subscribing to their feed. You can also page through his slides as you listen.