While not specifically related to caching in PHP applications, this recent post from developer.com has some good reminders of how much the right kind of caching can help your application to really fly.
If you're looking at performance and you want to get some quick wins, the obvious place to start is caching. Caching as a concept is focused exclusively around improving performance. [...] Fundamentally caching has one limitation - managing updates - and several decisions. In this article, we'll explore the basic options for caching and their impact on performance.
They talk about three different update methods - a synchronized update, lazy update and a read-through strategy (where the caching functionality itself can force a re-read of the original source). There's also a brief look at options to consider when caching data and how you're going to manage that cache once you've filled it with data.