Can a fellow geek knowledgeable in PHP gives me a few pointers?

I have inherited a 12 year old PHP app written by 2 interns. The code was written with Notepad++ (no IDE), no comments except when they copy pasted something from the internet, most variables are single letter and it's the biggest spaghetti bowl I have ever seen.

To add insult to injury I have no experience with PHP (apart from peeking at this code to fix a minor bug here and there). To be fair the app works OK and has done the job for all those years.

This app has never been updated so it's PHP version 4 or 5 I think, it runs on an old Debian 5 VM, it's not facing internet, internal use only. It was supposed to be deprecated last year, but now given our budget (or lack thereof) and the sheer incompetence of our new provider I don't see it happening soon.

Is there a way for me to have an IDE with a kind of "compiler" that would help me convert this to PHP 8 so I can migrate it on a more recent VM? Of course there's 0 budget for this.

Also worth checking out is a package called Rector (https://github.com/rectorphp/rector) which will automate a lot of this by scanning the codebase and providing diffs so you can upgrade safely.