Create secure user registration and integrate ibill.net

I need a safe and secure user registration form. The form needs to work with [login to view URL] and also store data into a MySQL database. I would like the form to send an email to have users confirm their registration. Then once the user is confirmed, they would goto the user only page and be able to buy credits through ibill and then ibill will post back the amount to the MySQL database. The website and database are already in place and only need the script. There will be defiantly be more work in the near future if this works out... Most importantly this project needs to be secure and easy to use overall