What is a good way to check if an image is unique using PHP?

Question

What is the best way to check if an image is unique using PHP? Say I have a directory of about 30 images (about 500*500 pixels), and someone uploads another picture, what is

Answer 1

Byte-wise comparison of files will fail even when a small detail like a ID3 tag has changed. To compare the picture contents, you would have to open the image file and create a hash of the actual image pixel data. But even that can be undone by saving, say, a JPEG file twice with a slightly different quality level - the subtle encoding differences will create changes in the pixel colour values.

So if you are really looking to match image contents across formats and qualities, you are opening a huge can of worms :)