In this new post, Justin Silverton shares a few helpful hints for creating high performance applications in PHP:

• use multi resultset queries to your database rather than many small ones
• page/object caching
• gzip compression
• tune your web server
• Don't save performance testing for the end of the project

Each of the items has explanation and links to more details on them (including links to three difference object caching applications - ion cube, alternative PHP cache, turck MMCache).