Well, PHP was once an abbreviation for Personal Home Page, but that changed over the years. Nowadays it is the abbreviation for "Php Hypertext Preprocessor," there is no-one however who will refer to PHP different than "PHP."
I personally always loved that they changed it from Personal Home Page to PHP: Hypertext Preprocessor. I have always been fascinated by recurrsive acronyms Truth is, defining PHP still doesn't tell you what PHP means because it's part of the acronym.