Watermarking with cache
Here's the more advanced version of transparent image watermarking using Apache mod_rewrite and PHP.
While that more basic version is good for purposes of showing general ideas, this script caches watermarked images so that they do not have to be generated with every request.
Caching mechanism checks file change dates and updates the cached file if change in the original file, watermark or script file happens.
Here's the output of the script working on this web site:
