Core PHP Programming, Third Edition

24.2. Controlling the Browser's Cache

One hassle of writing dynamic Web pages is the behavior of caches. Browsers maintain their own cache, and by default they will check for a newer version of the page only once per session. Some ISPs provide their own cache as well. The intention is to avoid wasteful retransmission of pages. However, if the content on your page potentially changes with each request, it can be annoying if an old version appears. If you are developing an e-commerce site, it can be critical that each page is processed anew.

On the other hand, your page may be dynamically building a page that contains information that doesn't change very often. My experience has been that caches are smart enough to store URLs that appear to be ...