On the Monitor.us blog there's a recent guide to performance tuning your PHP with sections on lots of topics, some related directly to the code and others more towards the environment it lives in.

The World-Wide Web offers more PHP performance tips than can be comfortably discussed in one article, so the following is merely a list that can be used for reference purposes. The tips are divided into categories to group similar things together and make it easier to find what we need. The author’s search for tips was extensive, but completeness still cannot be guaranteed.These tips fall into category #3.1 (the server executes a script) in the taxonomy of tips that we have been working with.Because of the sheer volume, these tips have not been tested. The reader is expected to test them in his own production environment before relying on them. This is not unreasonable, though, because the value of most tips depends on the unique environment within which PHP operates.

Sections in the guide include:

• Caching
• Session handling
• Compression
• Memory Leaks
• Code optimization
• Using services

Also included in the post are a list of links that were references for the tips in the list..