Change to web voucher system

Hi, I have a bespoke web shop that has a backend voucher system - It generates a code and gives a $ value discount off the shopping cart. I need it changed to give a % discount off the shopping cart. 1. I generate a code that gives a % discount 2. When a user is checking out they can enter in the code and receive the % discount. The shop is made in PHP, MYSQL, JavaScript. You will have root access to the test server. Payment on project completion - Experienced people only please. Real urgent, need it completed before the 2nd of December. Thanks