On the Web Development Blog Olaf helps to answer the question "why should you cache your PHP website?" with a few helpful tips, bits of code and pointers to other tools.

Most web servers are able to handle "normal" traffic and there are plenty of websites which doesn’t have so much traffic. So maybe you ask yourself: Why should you cache your PHP powered website? The apache web server is able to serve many, many files at the same time, but all these files need to be static. A PHP script is parsed by the web server and next the generated HTML data is send to the client (web browser). While this happens the server need to use much more memory than by sending a file to a web client. Imagine what happens if you run/parse a page which is build with WordPress...

He points out the WordPress Super Cache plugin and mentions that there's lots of other tools out there in the form of plugins or things like eAccelerator. He even includes a simple custom caching script that grabs the contents of a remote file and caches it locally for twenty-four hours.