PHP Header function detailed

The caching of the Web page is controlled by the "Cache-control" in the HTTP message header, the common value has private, No-cache, Max-age, must-revalidate, etc., the default is private. Its role is divided into the following situations according to different ways of re browsing:

(1) Open a new window

Value is private, No-cache, and Must-revalidate, the server will be accessed again when a new window is opened.

If the Max-age value is specified, the server is not again accessed within the time of this value, for example:

Cache-control:max-age=5 (for 5 seconds after accessing this page will not go to the server)

(2) Enter in the Address bar

A value of private or must-revalidate accesses the server only for the first visit and is no longer accessible.

Value is No-cache, then it is accessed every time.

Value is Max-age, the access is not repeated until it expires.

(3) Press the Back button

Value is private, must-revalidate, and Max-age, it will not be accessed again.

Value is No-cache, the access is repeated each time

(4) Press the Refresh button

Regardless of the value, repeated access

When the Cache-control value is "No-cache", accessing this page does not leave a page backup on the Internet temporary article folder.

In addition, caching can also be affected by specifying a "Expires" value. For example, to specify that the Expires value is a long past time, if you repeatedly press ENTER in the Address bar when you visit the network, you will repeat the visit each time: Expires:fri, Dec 1999 16:00:00 GMT

For example: Prevent the page from being cached in IE

HTTP response message Header settings:

CacheControl = No-cache

Pragma=no-cache

Expires =-1

Expires is a good thing, if the Web page on the server often changes, set it to-1, indicating immediate expiration. If a page is updated daily 1 o'clock in the morning, you can set the expires to 1 o'clock in the morning the next day.

When the HTTP1.1 server specifies CacheControl = No-cache, the browser does not cache the Web page.

Legacy HTTP 1.0 Servers cannot use Cache-control headers.

So for backward compatibility with HTTP 1.0 servers, IE uses the Pragma:no-cache header to provide special support for HTTP.

If the client communicates through a secure connection (https://)/with the server, and the server returns Pragma:no-cache headers in the response,

Then Internet Explorer does not cache this response. Note: Pragma:no-cache only prevents caching when used in secure connections, and if used in unsecured pages, the page will be cached but marked as immediately expired if it is treated in the same way as Expires:-1

Header Common Instructions

The header is divided into three parts:

The first part is the HTTP protocol version (http-version);

The second part is state code (status);