Phi Phi Island weather is typically the same as that of Phuket weather. The best time to visit the island would be between November and March, in the dry season. This holds true for the majority of Thailand, so you can plan your entire trip in the same season without worry.
Typically September and October are the wetter months, being in Thailand’s monsoon season. So keep this in mind when planning your Phi Phi Island holidays as you might want to steer clear if you don’t like getting wet!