What factors make PHP Unicode-incompatible?

Question

I am able use UTF-8 characters just fine in my scripts.

As a matter of fact it is possible to have names of variables and functions contain Unicode characters.

Answer 1

Many of the common extensions do not have unicode support or (even worse) you "need to know" that a string contains unicode/utf-8 sequences, like for example XMLReader. And it can make quite a difference wether PHP's glob() calls FindFirstFileA or FindFirstFileW on win32.
Another (much smaller but surprisingly often being the source of annoyance) issue are BOMs which PHP do not recognize.