Phi is used in a lot of other places, so there's some ambiguity inherent in using that. $ is used in programming, but that's something that came after the symbol was assigned. ¥, £, € are not used for anything else that's common.
I don't agree. Whilst it is used for other things, it wouldn't be ambiguous for currency, as it is not an overloaded symbol in that context. The current symbol, a struck-through 'B', is the symbol for Thai currency - now that's ambiguous!
