I have two identical images stored in two different servers (dropbox and my own).
however, i realize that

hash("md5",file_get_contents($image1))
hash("md5",file_get_contents($image2))

are returning different hashes for these identical images.
How can i make sure they both return the same string?

Have you tried hash_file? to see if you get the right results.

I am not sure why it would give you different values though, have you tested it with other images files from different sources. I tried it myself with dropbox and local copies and hash and hash_file work as they are supposed to.

Thanks tony,

I looked at the images again, and they were not the same. One of the thousands of pixels had a rgb value that was off by one from the other image. I've been using hash md5 now and it works fine.

Is there a way to make hashes so that they dry similar images will have the same hash value? Or should I just use a hash encoder that returns a shorter hash string then, like something other than md5?

There are algorithms out there for that. Specially for image caching for search engines like google. I am not familiar with those algorithms, Maybe there is a library that you can use or port to PHP to use.

Edit: This is the closest thing I could find quick, probably a starting point.