Migrating Custom CMS Script from PHP 7 to PHP 8

Hello, a friend of mine programmed me a simple, small and leightweight CMS some years ago.

The script uses a Mysql database and has 22 individual files with the template files that do not need to be edited. In the Year 2019 the Script was successfully migrated to PHP 7.

Now i need the compatibility with PHP 8, but it should continue to be backwards compatible.
Make me an offer please.