Licence form to check previous licence in the DB -- 2

On our website to have a form for the clients that would like to renew our software. To create simple PHP/MySQL form that checks if clients that wants to renew licence has previously purchased licence with us. Form - AJAX or nice Java Script checks the licence entry in the DB. If match is yes: it will redirect visitor to the custom purchase page that is otherwise hidden if not: will ask to resubmit again or to send message to support for verification. Need this to implement on two webpages and checking the licence in the two databases tables, first from one, then from the second one. Online system is in place already. but this function for licence check from the DB was not implemented yet. I will provide access to the DB and to the FTP