Using PHP how can we tell if the image already exists on over server regardless of name?

I have seen several websites where if you upload an image and an identical image already exists on there servers they will reject the submission. Using PNGs is there an easy way to check one image against a massive folder of images?

http://www.imagemagick.org/discourse-server/viewtopic.php?t=12618

I did find this with imagemagick, but I am looking for one vs many and not one to one a million