PHP Anthology, Volume 2: Applications. Chapter 5: Caching:
For example, the content delivered by a dynamic page may change once a day, so there’s certainly a benefit to be gained by having the browser cache a page-even if only for twenty four hours.
It’s usually possible to live with both problems given a small PHP application, but as the complexity of, and traffic to, your site increases, you may run into difficulties. However, both these issues can be solved, the first with server side caching, the second, by taking control of client side caching from within your application. The exact approach you use to solve the problem will depend on your application, but in this chapter, we’ll see how you can solve both using PHP and a number of class libraries from PEAR.