For our website based on CakePHP we want to implement a gift coupon system where users can buy a coupon (via paypal) which is sent as PDF to the buyers mail.
The coupon has to contain a unique alphanumeric key that can be used to buy any offer on our page (which are all of the same value). So when a user wants to buy something on the page, he can choose between normal payment and using a giftcard. If he chooses giftcard a text field appears where he can input the coupon's number and after automated checking of the number the buying process is completed automatically.
The system has to be very secure since every coupon will be worth over $100.
Please ask if you have any questions.