Revised Project: php customization for Abantecart 1.2.13 (old project deleted)

I want to collect specific latest-order data from the abantecart db and send it to an entirely different db on the same server, and I want to accomplish this by adding code to an existing "Continue" button on a specific template page of the cart as a hidden background process. Milestones would be: (1) create new code for an existing button used on a template page to activate a custom db query. (2) write a mysqli database query to get all details of last completed order (MAXid): customer details, order details, order attributes details (table JOINS will be necessary); (3) create code to send the results of the query to a handler page that will reside OUTSIDE of Abantecart.