All you wanted and need to know about caching PHP applications in one article by Ben Balbo over at Sitepoint.

In the good old days when building web sites was as easy as knocking up a few HTML pages, the delivery of a web page to a browser was a simple matter of having the web server fetch a file. A site’s visitors would see its small, text-only pages almost immediately, unless they were using particularly slow modems. Once the page was downloaded, the browser would cache it somewhere on the local computer so that, should the page be requested again, after performing a quick check with the server to ensure the page hadn’t been updated, the browser could display the locally cached version. Pages were served as quickly and efficiently as possible, and everyone was happy.

Read on: Cache it! Solve PHP Performance Problems [PHP & MySQL Tutorials]