If you are at ApacheCon, then come see my talk on Wednesday at 1.30pm in Parthenon 4; it'll be fun. It's quite language agnostic, so it'll be worthwhile wether you do PHP, Perl, Ruby or something entirely different. Here's the blurp from the program:
YOU can play with the Big Boys and do thousands of dynamic requests per second. Some extra thought into the overall architecture beats the pants off any amount of micro-optimizations. You'll learn exactly how to do it.
Why horizontal scaling rocks. Why vertical scaling sucks. Which caching strategies work best when and why. Benchmarking techniques -- How to measure accurately. How to avoid scaling vertically on the backend. Scaling your database systems without shelling out for that gazillion dollar Sun box. Segment your data the right way when your database can't keep up. The when, where and what about session data. Manage your resources to get the most out of your hardware. Make a job queue to gracefully deal with big traffic peaks.
Ask Bjørn Hansen once architected and built the core of a major internet advertising system plastering ads on hundreds of thousands of screens every minute. The session will also touch experiences from Citysearch and the insane peak loads at Ticketmaster.com.
(that matches surprisingly well with what I am actually going to talk about, whee).
Perrin is doing his Building Scalable Websites with Perl which is a bit of the same, but Perl specific (for better or worse). If you do Perl, go see that one too -- it's good. :-)