I am planning to buy an iPhone in South Korea on October and i live in the Philippines. Will the iPhone work in my country? or do i need to do something to make it work in my country, Philippines?

You will need to make sure you buy an unlocked phone from Apple. As noted, the warranty will only be valid in South Korea. If you would need any warranty service, you will have to take it back to South Korea.

Best you buy iPhone where you live in Philippines.

Warranty and service are only valid in country of purchase,

As ckuan and kb1951 correctly told you, you will not have a warranty outside the country where it was purchased.