how to use control-cache headers?

Question

I downloaded Google speed tracer for Google chrome to see how my site does performance wise and it tells me I need to enable caching for certain files like my style.css, images, etc.

I've read that the below php code should tell browsers to cache html content. I wrote a quick php page with a couple of images on it and stuck the below code at the top (before the headers are sent) to test to see how it worked.

Header("Cache-Control: public, max-age=3600, must-revalidate");

When I go back to speed tracer's analysis it says...

Summary From Cache: false

Request Headers Pragma: no cache Cache-Control: max-age=0

but under Response Headers... Cache-Control: public, max-age=3600, must-revalidate (exactly what I specified)

I'm a little confused, what's going on...? When it says from cache: false does that mean from the server cache and not the client's cache?

Answer 1

The key is must-revalidate: This means, that the client is asking the server if the file has changed. If you don’t handle this case, the browser will fetch a new copy.

Read Mark Nottingham’s fantastic Caching Tutorial for more information. As an example for a PHP implementation you may use my code.

Look into $_SERVER['HTTP_IF_NONE_MATCH']and $_SERVER['HTTP_IF_MODIFIED_SINCE'] for validating clients. And be aware that both headers may contain malicious code. ;)