What is a subducting plate involved in the formation of volcanoes of the Philippine islands

Answer: Subduction of Pacific plate under Phillipine plate

and, subduction of Phillipine plate beneath the Indo - Australian plate

Explanation: The formation of volcanoes at the Phillipine region is because of the collision between the Pacific plate and the Phillipine plate at the eastern side of Phillipine plate, where the denser Pacific plate gets subducted beneath the Phillipine plate. Again towards the western and the northwestern side of the Phillipine plate, the Phillipine plate is getting subducted beneath the Indo-Australian plate.

Due to this constant collision between this three plates, the volcanoes have started to form at the phillipine island and it also lies in the region of the Pacific's ring of fire, so it is a tectonically an active region.