We've been using phash for an image board for a while now and are quite happy with it. We only use it to detect reposts when someone uploads an image. It gives some false positives quite often, but that's totally okay for our use case. We specifically set it up to err on the safe side. Users are only presented with a "Are you sure your upload is not a duplicate?" message.

Currently we're just doing a `WHERE BIT_COUNT(images.phash ^ inputHash) < 12` in MySQL over 400k rows, which still works reasonably well (~200ms) given that it can't use an index for the XOR/BIT_COUNT operation. To my knowledge there's no way to speed up this query in MySQL, so if we continue to grow we probably have to write a small daemon that is able to search hashes more efficiently.