Over at Sitepoint Christoper Pitt shares some research he has done in finding differences in images using PHP.

I recently stumbled across a fascinating question: how could I tell whether an image had changed significantly? As PHP developers, the most troublesome image problem we have to deal with is how to resize an upload with an acceptable loss of quality. In the end I discovered what many before me have – that this problem becomes relatively simple given the application of some fundamental mathematical principles. Come along with me as we learn about them…

https://www.sitepoint.com/finding-differences-in-images-with-php