I was able to convert a PHP 5.4 app to 7.1 but after that there are too many backwards incompatible changes to be easy to fix. Even the upgrade to 7.1 took a few days of tweaks to the code.
In my case the upgrade was demanded by security because the latest PHP5 release is no longer maintained.
For an internal app you're much better off just leaving it on PHP 5. Maybe add an op-cache for performance.
I know PHP is interpreted hence my comment "to have an IDE with a kind of "compiler"".
What I would like is an IDE experience: everything that isn't PHP8 compliant is underlined in red so I can fix the thing without launching the app every time and see if I broke something.
Compiler: you are looking in the wrong direction, PHP is interpreted. There were some compilers, but it's not what you need.
Converting to PPHP 8: do you need that? what are you trying to achieve? You can keep the app another year or so it it is working, don't fix what is not broken. There is no way to convert to 8, you need to rewrite it, PHP become more object oriented with each version and that is a paradigm shift there is no converter for.