On Phi Phi most of the larger resorts will take credit cards for your room bookings, but for most of the smaller ones and definitely for purchases around the island, restaurants, drinks etc cash is king. But not to worry because there are lots of ATM's around the island avialable 24hrs. You should check with your bank what the charges are, the Thai charges for cash are small but often your bank or credit card company back home will apply a charge for foreign transactions. As a rule of thumb it works out cheaper to take out the maximum rather than small amounts as charges tend to work out less.