I don't mean mainland China, I mean Taiwan. The Philippines has made agressive moves toward Taiwan in the past for some reason despite the fact that the ROCN has individual warships that could single-handedly wipe out the entire piss-poor Filipino excuse for a "navy".
First, who the hell is the US going to side with? The US always likes to uphold both as some kind of bulwarks against evil Chinese commies. My bet is that the US would side AGAINST Taiwan in that case, but there would be significant political fallout. I'm not sure if the US would want to arm the Philippines since Taiwan and the US have too big of an economic stake in each other.
Second, there are significant numbers of Filipinos living on Taiwan as domestics, prostitutes, or mail order brides. What would happen to them? Mass deportation, confiscation of assets, or internment camps? I suspect the ROC government has not even considered what to do in the event of conflict.