Disable cache for some images

Question

I generate some images using a PHP lib.

Sometimes the browser does not load the new generated file.

How can I disable cache just for images created dynamica

Answer 1

Solution 1 is not great. It does work, but adding hacky random or timestamped query strings to the end of your image files will make the browser re-download and cache every version of every image, every time a page is loaded, regardless of weather the image has changed or not on the server.

Solution 2 is useless. Adding nocache headers to an image file is not only very difficult to implement, but it's completely impractical because it requires you to predict when it will be needed in advance, the first time you load any image which you think might change at some point in the future.

Enter Etags...

The absolute best way I've found to solve this is to use ETAGS inside a .htaccess file in your images directory. The following tells Apache to send a unique hash to the browser in the image file headers. This hash only ever changes when time the image file is modified and this change triggers the browser to reload the image the next time it is requested.

FileETag MTime Size