Pragma: no-cache

PHP Advanced and Object-Oriented Programming

Larry Ullman

 

Last-Modified 最后修改时间

Expires 过期时间

Pragma 编译提示

Cache-Control 缓存控制

如缓存系统发现Last-Modified的值比页面缓存版本的时间更接近当前时间，它就知道应该使用来自服务器的更新页面版本。

 

Caching—both in Web browsers and proxy servers—can be affected using PHP’s header() function. Four header types are involved:

• Last-Modified

• Expires

• Pragma

• Cache-Control

The first three header types are part of the HTTP 1.0 standard. The Last-Modified header uses a UTC(Coordinated Universal Time) date-time value. If a caching system sees that the Last-Modified valueis more recent than the date on the cached version of the page, it knows to use the new version from the server. Expires is used as an indicator as to when a cached version of the page should no longer be used (inGreenwich Mean Time).

Setting an Expires value in the past should always force the page from theserver to be used:

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

Pragma is just a declaration for how the page data should be handled. To avoid caching of a page, use

header("Pragma: no-cache")

The Cache-Control header was added in HTTP 1.1 and is a more finely tuned option.

Directive       　　Meaning

public   　　　　　Can be cached anywhere

private         　　Only cached by browsers

no-cache        　 Cannot be cached anywhere

must-revalidate  Caches must check for newer versions

proxy-revalidate  Proxy caches must check for newer versions

max-age   　　　A duration, in seconds, that the content is cacheable

s-maxage  　　　Overrides the max-age value for shared caches

Keep all systems from caching a page

header("Last-Modified: Mon, 26 Jul 2016 05:00:00 GMT"); //Right now!

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); //Way back then!

header("Pragma: no-cache");

header("Cache-Control: no-cache");