XMLWriter

that is the way in which i'm trying to load it, as if i was loading the PHP extension, but it is not working, i don't know why CI4 thinks that i'm trying to use a CI4's class, i already reviewed the extension in the server and it looks like that extension is enabled and running, any help/hint will be appreciated, thanks.

God. My bad.
XMLWriter is the default php class.
Use a backslash before the class name. Like \XMLWriter.
This means that the class is in the global namespace.