Aditya Agarwal, Director of Engineering at Facebook, gives an excellent presentation on the architecture of Facebook with a focus on the LAMP (Linux Apache MySQL PHP) based components:
Facebook: Science and the Social Graph
The numbers are very impressive!
I have been writing about web technology and software development since 2001. I am the developer of the Alpha Framework for PHP. I love open source, technology, and economics. You can follow updates from this blog on Twitter.