F5/refresh button and forward button issue after using POST method in a form

Question

I read the answer here Simple Post-Redirect-Get code example, in my code, this works only when I clicked/pressed the back button. This is the process of the code

1. POST
2. Validates and if passed insert the data to database
3. Redirect them to the same page Redirect::in($_SERVER['REQUEST_URI']) //I applied the accepted answer on the link here.

Problems are:

• F5/refresh = confirm submit form is appearing.
• Forward button = Document expired.

How can I solve this problem? I am aware/search of PRG pattern but failed to fully understand it and tried the accepted answer on the link but I still have some problem.