I have recently moved this blog to be hosted on Fastly, a CDN service with bunch of great features like dynamic content caching with instant purges. Fastly utilizes HTTP headers to determine what to cache as described in the Cache Control document. While configuring my service I noticed that my Wordpress (origin server) kept returning HTTP headers like these
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
I looked through the Wordpress code and couldn’t see where such value was set. After some internet searches I discovered that they are set using session-cache-limiter option in php.ini. In most distributions this defaults to nocache which ends up with above headers. You can read more on the cache-limiter options here.
http://www.php.net/manual/en/function.session-cache-limiter.php
What we need is session-cache-limiter = public results in headers like these
Expires: (sometime in the future, according session.cache_expire)
Cache-Control: public, max-age=(sometime in the future, according to session.cache_expire)
Last-Modified: (the timestamp of when the session was last saved)
e.g.
Expires: Tue, 22 May 2012 16:38:33 GMT
Cache-Control: public, max-age=10800
Last-Modified: Tue, 04 Oct 2005 00:55:59 GMT
If you want to adjust the max-age you can set cache-expire in php.ini e.g.
; http://php.net/session.cache-expire
session.cache_expire = 180