It's a recursive acronym. The first P stands for PHP, the H stands for Hypertext and the final P stands for preprocessor http://en.wikipedia.org/wiki/Php
In 1994 when Rasmus Lerdorf created this language for personal purpose PHP meaned Personal Home Page but now and after all the updates and the popularity of this language PHP stands for Hypertext Preprocessor
i am sure, in past it was PHP: Hypertext Preprocessor.. But now i am sure this abbreviation has been deprecated. Now PHP is an abbreviation of nothing.. PHP is just PHP.
In the past it was originally Personal Home Page/Forms Interpreter, and as of PHP3 it's the recursive acronym