When are you planning to fly?
Kayak.com is a good place to start. It's likely to be cheaper to buy the two separate roundtrip tickets, one from Europe to Manila and another for the domestic flight from Manila to Davao. A quick search for some dates in March finds roundtrips from CDG to MNL on China Southern (changing planes in Beijing) for about $646 USD roundtrip. From MNL on to DVO, roundtrips on Airphil Express seem to be about $114 USD.