[Bish0pQ]

Just link it to a database? Wouldn't be to hard, add 3 columns in your user table, inv_code1, inv_code2... and when a user tries to sign-up check if the key is found in the database? For generating the keys you could either use your own algorithm or use an existing one, a personal one would be best in terms of security (if you know a little crypto) but I'm sure people here can help you explain that.