Since, Phi Phi Islands does not have its own airport; the only mode of transportation is by boat. You can travel by sea, the nearest ports to reach Phi Phi Islands include Phuket and Krabi. You can either take long tail boats, speed boats or ferries.
If you are planning to take a boat from Phuket it will take around two hours to reach Phi Phi Island. And if you are planning to take a boat from Krabi it will take around one and a half hours to reach Phi Phi Island.
You can reach Phuket by air, since it has its own international airport, where flights are connected from around the world.