For most folks, PhD means "Doctor of Philosophy". But, for Jorge Cham, it means "Piled Higher and Deeper". Can you explain to me the title of that comic book?
"PhD" is an abbreviation. The title of the book is playing around with this abbreviation. Although normally understood as you said to stand for Phillsophiae Doctor, the letters do not actually state this.
It's for example, BMW, usually understood to mean Bavarian Motor Works , but it doesn't actually state "Very Important Person". Anyone could come along, and then play around with the letters and make it "Big Mouth Woman", or "Bouncing With Me."
It's a kind of pun