It was one-way traffic throughout but Japan were kept at bay by Philippines goalkeeper Nina Meollo until just before half-time. Mina Tanaka broke the deadlock with a scrappy close-range header before Koga doubled the lead from a corner.
Japan accelerated after the break as Remina Chiba struck a right-footed shot in the 65th minute to make it 3-0.
Two minutes later, Manaka Matsukubo dribbled through the defence to add a fourth before Koga scored her second with another close-range header in the 75th minute.
Momoko Tanikawa added the sixth in the 86th minute and substitute Riko Ueki scored the seventh in the 90th to seal a dominant win for Japan, who have now scored 24 goals without reply in the tournament.
With the win, Japan booked a spot at the 2027 Women's World Cup in Brazil, while the Philippines will compete with the other losing quarter-finalists for two more places in playoffs on the Gold Coast.