talks.php.net is a little known part of the php.net site which features most of the presentation slides from the PHP gurus' speaker sessions. Even though the slides will not always tell the complete story (and were obviously designed by nerds) they are are still a good source for tiny optimizations that can help you to improve the performance of your server or your scripts - especially if you run your own server and have access to the Apache configuration file and the php.ini settings. But there are also coding hints and database access tips that are helpful if you can only optimize on the script level.

For a starter I recommend you to take a look at Ilia Alshanetsky's Performance Tuning PHP.