DeveloperTutorials has this article posted today about "PHP Caching to Speed up Dynamically Generated Sites".

This entire site, like many, is built in PHP. PHP provides the power to simply 'pull' content from an external source, in the case of my site this is flat files but it could just as easily be an MySQL database or an XML file etc.. The downside to this is processing time, each request for one page can trigger multiple database queries, processing of the output, and formatting it for display... This can be quite slow on complex sites (or slower servers).

To the resuce are the caching abilities that PHP has to offer - output buffering, both to a simple varaible and to a local file - allowing you and your site to see a performance boost. He gives some example code to create this caching technique, so it's pretty simple to drop into your pages. It's not the most powerful solution, but it's a nice, quick alternative...