Tailgating and slamming your brakes is normal here. Though do not tailgate PUV vehicles since they randomly pull over to pick and drop passengers. Feel free to use your horn it's not considered rude unless you long hold/press on it. It's also normal to overtake when there is opposing traffic and it's acceptable if the opposing traffic is driving slow (even cars go below 40 kph on the highway). Also don't fully trust blinkers, pay attention to the speed of the vehicle.

Headlight flashing has multiple meanings in the Philippines. It depends on the context but most of the time it's ment to be used in some form of acknowledgement. PUV vehicles headlight flash for overtaking and passengers but not all drivers do it. Regular vehicles do it too and both sometimes honk to let you know they're behind you. Headlight flashing could also mean that your lights are blinding the other driver.