HTACCESS image caching rule that checks the image modification time - php

Question

I'm serving images two different ways:
Using a PHP script for profile pictures for example
By pointing to them directly, for icons and backgrounds for example
I'm in the process of handling their caching properly, and i'm totally new to this.
For the PHP script, i'm just adding a Last-Modified header to the response, and delivering a 304 status code if it's called again, if the file hasn't changed (using filemtime()).
For direct accesses, i'm using HTACCESS, but every rule i saw so far doesn't allow me to do the same as in my PHP script (checking if the file has changed, then serving a 304 or the file itself).
Here's the HTACCESS rule i'm planning to use:
Header unset Pragma
FileETag None
Header unset ETag
# cache images/pdf docs for 10 days
<FilesMatch "\.(ico|pdf|jpg|jpeg|png|gif)$">
Header set Cache-Control "max-age=864000, public, must-revalidate"
Header unset Last-Modified
</FilesMatch>
From what i understand, the only way of updating a cached image is to rename it. Does someone know a way around it? By checking the image's last modification date for instance?

Answer 1

You could use mod_expires, if available:
<FilesMatch "\.(ico|pdf|jpg|jpeg|png|gif)$">
ExpiresDefault "modification plus 10 days"
</FilesMatch>

Answer 2

What you are doing with PHP should do apache automatically for static files. It will set the Last-Modified header and respond with 304 if it will find if-Modified-since in the request. This is done automatically and has nothing to do with caching. It will not prevent repeated requests to your server, it will just save you bandwidth (and loading times for user) when the file is not modified by returning just 304 info instead of the whole file.
To prevent those repeated requests to your server, browser (and proxy servers) has to do some caching. You can control the caching either via HTTP headers or for HTML also via META tags. When you specify that the file is cacheable for 1 week, browser won't try to contact your server for 1 week (although most browsers are set to revalidate cache entries on first access after startup).
So you will either live with the possibility that some users will use old cached copy for some time (depends on the expiry header) or you must change your URL as Gerben suggested. Only then you can be 100% sure that everyone will get the new version (this is important for javascript as having some of the js files old and some new can make very strange errors). Nowadays almost every high performance website uses the file.ext?v=3 approach, so that they can set the expiry header to large values like 6 months.

Answer 3

As #Gumbo pointed out, "Apache should already do that for static files".
And that's true, Apache does that, so that kind of stuff works fine:
<FilesMatch "\.(ico|pdf|jpg|jpeg|png|gif)$">
Header set Cache-Control "max-age=864000, public, must-revalidate"
</FilesMatch>
ps: Sorry #Gumbo, but i asked you to change your answer so that i can accept it, but you wouldnt do it and i had to close that question eventually, so.