This is an abbreviation that asks the question, as stated above, what Niel Patrick Harris would do in x situation. So, basically, if something is going down, one would ask WWNPHD?
"I don't know, but WWNPHD?"
"Chill, man. WWNPHD?"
Luc: "I'm screwed."
Luc: "He'd use the Force!"
Obi-Wan: "You have learned well."
Kumar: Relax! Wwnphd in a situation like this?