John Lim has updated his Optimizing PHP article with some new tips on caching data:

In this chapter, we explain why optimizing PHP involves many factors which are not code related, and why tuning PHP requires an understanding of how PHP performs in relation to all the other subsystems on your server, and then identifying bottlenecks caused by these subsystems and fixing them. We also cover how to tune and optimize your PHP scripts so they run even faster.

The article talks about true high performance, some of the bottlenecks that can keep you from it, tuning your Apache/IIS web server, tips on caching and some ways you can benchmark your application to figure out the best configuration for you.