How do I tell if someone's faking a filetype? (PHP)

Question

I\'m programming something that allows users to store documents and pictures on a webserver, to be stored and retrieved later. When users upload files to my server, PHP tells m

Answer 1

Many filetypes have "magic numbers" at the beginning of the file to identify them, You can read some bytes from the front of the file and compare them to a list of known magic numbers.