[Zamaroht]

First you need a hashing function, which recieves a string as parameters, and returns a new encrypted string. The new encrypted string is what you would store in your database.
When the players try to log in, you ask them to input a password. You apply the hash function to the password they wrote, and compare if the hash is the same to the one you have stored. If it is, then the login was succesful, if it wasn't, then not.