PHP —

Caching

Most user agents, such as web browsers, allow HTTP responses to be cached, which can significantly reduce the number of requests sent to a web server and thus improve the performance of a web application. HTTP responses are cached by saving a response to a request in memory. When a browser considers a request, it first looks to its local cache to see if it has an up-to-date copy of the response before sending the request to the web server. Consider a web site that includes a company logo on the top of each page in an image element such as:

<img src="/images/logo.gif">

When the browser requests a page that contains the image, a separate request is sent to retrieve the image /images/logo.gif. If the image resource is cacheable, and browser caching is enabled, the browser saves the response in a cache. A subsequent request for the image is recognized, and the local copy from the cache is used rather than sending the request for the resource to a web server.

A browser uses a cached response until the response becomes stale, or the cache becomes full and the response is displaced by the resources from other requests. The primary mechanism for determining if a response is stale is comparing the date and time set in the Expires header field with the date and time of the host running the user agent. If the date and time are incorrectly set on the user agent's host, a cached response may expire immediately or be cached longer than expected. HTTP/1.1 is more sophisticated than HTTP/1.0 in controlling the life of a cached response using other parameters not discussed here.

Not all responses are cacheable. HTTP describes the conditions that allow a user agent to cache a response in some detail; essentially most responses are cacheable unless a cache-control header directs otherwise. There are many situations in which an application may wish to prevent a page from being cached, particularly when the content of a response is dynamically generated, such as in a web database application.