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.