Martin, one of our system architects, has written an article on our corporate blog about scalability in PHP.

He covers things like frontend generation (as opposed to request based caching), database replication, loosely coupled components to increase the scalability of the application, and he even mentions SOA (Service Oriented Architecture). We've recently had some good results with implementing SOA architectures in PHP. Traditionally SOA is associated mostly with Java, but with PHP5 it's very well doable.

Read his post here.