Need create a iPhone application, which basic is a web browser with QRCode Read function.
Basic it need able to allow user to log-in by access backend service which need to be created by PHP.
once user login, user could scan QRCode and send information to backend server process with PHP&MySQL.
and information send back to iPhone, it could be think as a WEB Page. which could show purchase information, system message etc to iPhone user.
so main function need to be done by HTML / PHP / MYSQL, on iPhone side just a WebBrowser.