I don't think busking in the Philippines is illegal, on a national level at least. The Revised Penal Code does not have a crime to cover this. Technically speaking, one is just playing music. There is a right called freedom of expression.
On the city level, there could be some local ordinance banning such sort of thing. But you can get a way with it by saying that you are only practicing your craft. You are not selling nor pandering. If someone gives you a tip, well, that's generosity.
If you can do Tai chi in the park or do HHWW in baywalk (both are modes of expression), the logic should follow playing the guitar.
The problem is now where to find a plug.....