Green’s Dictionary of Slang

1. (US teen) a fig. ‘degree’ held by a teacher who dislikes students who espouse ‘gangsta’ culture; thus used of one who undermines the activities of a ‘player’ [abbr. player-hater under player n. + SE degree].

2002		Teen Lingo: The Source for Youth Ministry 🌐 PHD An acronym/euphemism for a teacher who hates ‘gangster-like’ students. A Playa Haters Degree. A teacher that is constantly nagging a student is said to have a PHD.
2003		Eble Campus Sl. Nov.

2. (US black) a large penis [abbr. pretty huge dick, i.e. dick n.¹ (5)].

2004		Kanye West ‘Breathe in, Breathe Out’ 🎵 I always had a P.H.D. – a pretty huge dick.
2007		Skins ser. 1 ep. 1 [TV script] Well, the Ph D thing. Yeah, yeah, you could say that. You could say, um, pretty huge dick.