The Oracle Technology Network has posted the first article in a series by Eli White looking at building a scalable PHP/MySQL web application.
Hopefully the most important lesson you can learn here is to understand what you will need to do to scale in the future. By knowing this, you can do only what you need at each phase of your project without "coding yourself into a corner", ending up in a situation where it's hard to take the next scalability step. [...] In this two-part article I will share some of the lessons learned, and take you step by step through a standard process of scaling your application.
He touches on a few different topics in this first part of the series - performance vs scalability, tuning your PHP installation and database load balancing through master/slave replication.