In the README for this project, I see this: New to FireFS ? From console to web apps, you can read the wiki to know how to use this library into your project... but that "wiki" is empty.
Searching the web, I found this example program https://dev.to/na2axl/how-to-create-a-file-watcher-with-php-and-the-elementary-framework-1po, for which the "onAny" function seems to work. But the onCreated, onDeleted, onModified functions are not called at all in my environment (MacOS 14.2, PHP 8.1).