What does header("Expires: 0") do? [duplicate]

Question

Can anyone explain this?

Answer 1

Generally used to prevent caching. However, this document will probably provide you with a better explanation:

http://download.oracle.com/docs/cd/E13158_01/alui/wci/docs103/devguide/tsk_pagelets_settingcaching_httpexpires.html

To quote:

Never use Expires = 0 to prevent caching. The Expires header is sent by the remote server and passed through to the browser by the Portal Server. Unless the time on all three machines is synchronized, an Expires=0 header can mistakenly return cached content. To solve this problem, set the Expires header to a fixed date that is definitely in the past.

Answer 2

How caching works (among other things) is that you send a header to the browser telling it when the page's content will expire. This means that if you send a header like:

header("Expires Sunday June 10th 2011"); // not correct timestamp

The content won't 'expire' until that date. The browser can then use caching techniques to serve the page locally from your browser instead of having to download all the content again.

When you set the expiration to 0, it ensures that the next time the browser loads the page, it will download the content, thus giving you up to the second data. However, as you might see from other answers, you shouldn't send 0 to prevent caching, but instead send a date that is in the past.