Which one to use : Expire Header, Last Modified Header or ETags

php apache2 etag last-modified mod-expires
Avinash picture Avinash · Mar 16, 2011 · Viewed 26.3k times · Source

I am running PHP on Apache, and am confused about how to implement server-side caching, in order to make the site load faster.

What is the difference between the Expires, Last-Modified and ETag headers, and which one should be used in what situation?

Answer

hienbt88 picture hienbt88 · Mar 5, 2014

Expires and Cache-Control are "strong caching headers"

Last-Modified and ETag are "weak caching headers"

First the browser checks Expires/Cache-Control to determine whether or not to make a request to the servers.

If it has to make a request, it will send Last-Modified/ETag in the HTTP request. If the Etag value of the document matches that, the server will send a 304 code instead of 200, and no content. The browser will load the contents from its cache.

I recommend using one of the strong caching headers, along with one of the weak caching headers.

See also:

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