I have a private (non-internet) web-app that's still on PHP6.5; I like the new PHP features, but upgrading private code to conform to PHP 7 and 8 looks a lot like make-work. I wish it shipped with good upgrade tools.
[Edit] @darkwater (below) is correct; I meant PHP 5.6. PHP6 was some kind of abortion - I can't remember the story.
I'm doing this quite often as I maintain some code-bases that have backward compat down to PHP 5.3 as the baseline. So if you're looking for forward compatibility of your PHP 5.6 codebase to 7.x and 8.x (incl. 8.2), its not so much of work.
If you need an upgrade tool, have different PHP versions at hand and load each file on the CLI and PHP will show you the deprecation warnings. It then depends a bit on the code (each()/next()/reset() family undergone some changes if not removals in 8? so you have to replace with some different syntax) but most of the rest is only deprecation warnings that can be overcome with #attributes "annotations" that are just comments prior php 8.
tooling could be better especially if you're running from low versions, thought. but using php on the cli directly brings you quite far and on speed already for most things in my personal experience. git, find and xargs are your friends.
IIRC, the initial development was in the mid 2000s but it was abandoned several years later because they couldn't get Unicode support working correctly. By the time it was abandoned there was already a lot of material referencing it like books and conference talks. So when the PHP team decided to do a new major version most of the planned PHP 6 features were already added to 5.3 and 5.4 so they thought it best to just skip 6 to avoid confusion.