1. days before your flight, try to change your (PHP) to USdollar
2. then again, get about 30% of the USD and change it to WON (for pocket money)
3. the remaining 70% USD, just change lang to WON when you get to seoul, Preferably NOT on the airport.
try in myeongdong, nandaemun, other districts or one nearest to your place. pero better canvas first kung asa ang best rates offered. Dali raka kita ug Currency Exchange shop ngadto. If you need help just look for a logo with letter " i " that is Tourist Information, naa rana sila offices sa streets or find a TI personnel wearing red with the logo like this picture

they speak really good english !