> 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.

There is your problem. If it's runs on PHP5 (<?php phpinfo(); // in a php page), it might run without hurdle on more recent versions of PHP... or not, but PHP8 isn't a different language, they just added some stuff on top of PHP5. C extensions might break though, because yes, PHP relies heavily on them as the language is too slow to do anything meaningful like writing a database driver.