Convert PHP Code Application from 4.* PHP to PHP 8

I have an online personality test written in (I think) PHP 4, which no longer works. I think this is because my server no longer supports PHP the lower levels of PHP, or it throws up errors. I want the code converted to PHP 8.

I have attached the files so you can see the size of the task (you can ignore the PHPMailer as I don't use that).