It's not a surprise that ISIS could easily enter the Philippines. Why? Government supports them just to lead a rebellion against the current administration. At first, Maute, Abu Sayaff and NPA are the one terrorizing the south part of the Philippines especially in mindanao. This group cannot operate without sponsors. Philippines is already under Siege way back Marcos terms. It just so happen that this ISIS can caused a lot of damage to a small country like Philippines.
Philippines is not a super-power, and it is currently unable to prevent the ISIS members and funds from entering that country. The Muslims from Mindanao and the neighboring islands are having a long history of conflict against the Philippine authorities. And this is not the first time that they are receiving support from the outside players such as the ISIS.