An acronym used to shorten the common phrase online: Pics or it didn't fucking happen. While the origins of the phrase itself are believed to be derived from the group anonymous, the acronym was coined by a slightly unknown internet user known as neodudecurt. It didn't become a part of anything near common usage until July of 2009.
Actual letters used are capitalized:
Pics or It Didn't Fucking HAPpen
Boy #1: I had sex with the hottest girl last night
Boy #2: PIDFHAP!!!!!