I am creating a plugin. In this plugin I have shortcodes that provide some logic and content for pages. This way I have created 'Page A' and 'Page B'. Both of the pages will redirect user to external page. The external page is not written by me and will not process any of my $_GET or $_POST values. Then this external page will redirect user back to my plugin 'Page C'. Now how to make 'Page C' know if the external page was reached from 'Page A' or 'Page B'? Normally I would use sessions, but I have read that wordpress does not support sessions. How can I do it then?