Having a global state like this (let's call it "unmanaged" global state) is considered evil in many programming languages. It's mostly related to predictability: if a function relies on some piece of global state, it becomes less predictable since other parts of the application can change that piece of global state.

However, there are way to have some kind of global state in a "managed" way. For example Redux pattern, as shows in a React application here: https://plainenglish.io/blog/managing-global-state-with-react-redux

Indeed. I could answer this from a general programming perspective, but not from a PHP-specific perspective. Mostly it is about program design no matter language. But different languages have different scope rules and namespace rules, meaning that it may be more or less serious namespace pollution in some languages. Similarly, thread-safety is yet another concern but multi-threading is also somewhat language-specific.