PHP: Redirect to previous page with same session values

I'm currently creating a HTML/PHP based Attendance System. There's this page that I made for recording attendances; registration.php.

Now in registration.php, I have an event selector: - The Admin can change the event in which to record attendances. Let's say I want to change the current event which is "Event Name" (the default no value event) into Research Congress, the page will change its label and event value to "Research Congress";

Once I enter a student's ID and it's successful, I want it to redirect to the registration.php Index with the event as "Research Congress", not "Event Name". Basically, when I try to do this, it ALWAYS goes back to "Event Name" and not "Research Congress". How would I go about in coding this?