Getting a "unique identifier" for a file

Hi ! I'm trying to develop something in PHP. I have a list of file, and I want to know if I can have a unique identifier for each files. What I mean, is: I have my files: test.txt test2.txt I would like them to return a unique something... anything... If I have my first test.txt, and delete it, and put another one, EXACTLY the same file, but after deleting the first, I would like it to return a "new id" This "id" would be used to pair database entries for each files, but without using an old entry for a new file. I know it feels abstract, but if you want informations about what I want, ask me !