Programing a password or code system where a code can only be used a set amount of times

Intro: I want to be able to have a system where a person can purchase a code from my store and that code or password can be used on my PHP website. But each time that code is used the amount of given uses on that code will be decreed so when it has been used up the code will be no longer usable.

Example: My shop is selling a 5 use forum un-ban code.

The person goes and buys that code and is given a random generated diet code EX: 91259102

The person goes to the redemption site and enters that purchased code.

Once the code has been used that one time the code will only have 4 uses remaining on it.

Once the user has used the code 4 more times the code is no longer valid.

What I want to be able to do: Generate codes via a PHP website. (no need to be automatically generated)

Store the given codes on a .txt document on the server or using a MySQL database.

Have the user be able to use the codes on my website.

Final: Thanks for any input I can get it is all much appreciated.

Thanks, Code Squishy.