The boiling point of NH3 is higher than that of PH3 due to hydrogen bonding in the former. Unlike NH3, PH3 molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of NH3 is higher than PH3.
Well ,NH3 Has a Higher EN than PH3 and will exhibit Hydrogen bonding. Because of this, NH3 has a stronger bond with PH3 ,so NH3 needs more temperature than PH3 to break bonds,thus NH3 has higher boiling point,hope this helps,best of luck=)))