"Phbbbt" is the sound you make by putting your tongue between your lips and blowing. It is used when trying to show adverse feelings towards someone.
It is used mostly in instant messaging.
fun fact: in Yahoo! Messenger it has a dedicated emoticon: >:P , it's like :P but with a '>' in front of it, representing angry thoughts towards someone while sticking your tongue out
a: haha, my grade is higher than yours
a: you fail
b: >:P (phbbbt)
b: bite me