Upgrade approx. 60 PHP files from 5.1 to 8.1

I currently run a series of interrelated PHP files on my website. The files were written in PHP 5.1. While they still function, support for this version of PHP ended in 2006 and few web hosting services will support them. Mine does, for now, but looking ahead, it may be necessary to rewrite the code in PHP 8.1—and better to do it now, while the old PHP is still working and there's no urgency. The PHP performs two basic functions: 1. It generates and displays web pages (like the pages that open when you follow the links at [login to view URL]) from data entered in a control panel. PHP functions also permit the editing of these pages 2. It manages the creation and maintenance of subscriber databases (MySQL). 3. It authenticates user-entered credentials (including manually entered passwords or automatic IP address recognition) against the MySQL database in (2) and granting full text access after matching user-entered credentials against the MySQL database. In all there are about 60 different PHP files, ranging in size from 1KB to 17KB that perform these functions. All are written in PHP 5.1 and all need to be rewritten in 8.1, tested for functionality, and debugged if necessary. I've attached a sample file, [login to view URL], so that you can see the coding.