i just want to know a general information about this particular information. Any good reference will be helpful.
Image caching manual:
You can put image data into a cache and you might be able to gain some speed advantage from it over including them from the filesystem. However, you can and should also utilize browser and proxy caching, which makes more sense to me:
and maybe some webservers which utilize memcache:
Since the question was not limited to image caching though and asked for PHP, here is a list of general (opcodes) caches that you can use from PHP
Zend has a good caching tool if you want to check it out but I'm not sure about the image cache.
You could also check this link for a related stuff :