It is pronounced basically as if the H were not there. When the Thai language gets transliterated into English, H's show up everywhere but they are often silent, mere accent marks to soften a sound but not change it fundamentally. Thus Phuket is roughly Poo-ket, not Foo-ket. Same for Thai: pronounce it like the H isn't there but make an effort to soften the T. It's most like the first T in "tooth" and not at all like the TH in "tooth."