I need a php mysql script written to modify an existing signup process

General information for the website: I need the email authentication process of my moodle to be modified so that a user has to use a 'voucher code'
Kind of development: Customization of existing website
Description of requirements/features: Currently when a user self enrols into a moodle course the following process applies:
1. User completes sign up form
2. moodle sends email to user
3. user verifies self by confirming link in email
4. user enters course enrolment key to join course

As the enrolment key is a single key that can be used by many enrolees what I'd like to do is to have unique, use once only enrolment keys.

I'd therefore like to modify the self enrolment script so that when an enrolment key is entered, a check is made on a list of acceptable 'keys' for that course, if the key is in the list and it has not yet been used by someone else to enrol in the course, then the user should be enroled, and the key marked as 'used'.
Extra notes: I can provide copies of the existing script and access to the moodle for the selected pph'r.