When people talk about global variables in other languages it means something different to what it does in PHP. That's because variables aren't really global in PHP. The scope of a typical PHP program is one HTTP request. Session variables actually have a wider scope than PHP "global" variables because they typically encompass many HTTP requests.
Often (always?) you can call member functions in methods like
preg_replace_callback() like this:
preg_replace_callback('!pattern!', array($obj, 'method'), $str);
See callbacks for more.
The point is that objects have been bolted onto PHP and in some ways lead to some awkwardness.
Don't concern yourself overly with applying standards or constructs from different languages to PHP. Another common pitfall is trying to turn PHP into a pure OOP language by sticking object models on top of everything.
Like anything else, use "global" variables, procedural code, a particular framework and OOP because it makes sense, solves a problem, reduces the amount of code you need to write or makes it more maintainable and easier to understand, not because you think you should.