Phil is a teacher and Doug's best friend. He is partly responsible for the disappearance of Doug.
Phil is a teacher bored with his monotonous life, and ventures to Vegas with his friends (and Alan) to have a wild Bachelor party. Along with the other members he must re-trace his steps in order to find the lost groom to be.
Near the climax of the first Hangover Phil comes to the realization that they are not going to find Doug, and so calls Tracy, Doug's fiancée, to tell her the bad news. This is when they come to the realisation of where Doug may be, after a discussion on 'roofies'.
It is ultimately revealed that Phil, Stu and Alan moved Doug's mattress onto the roof, as a prank.