PHP is one of the most commonly used programming languages, borrowing from C and other languages to make a server-side language that was easy to use yet broad enough to scale.
Php started back in the mid to late 90's and has grown significantly in popularity and usage.
PHP has several possible meanings behind its acronym.
The first being Personal Home Page. While it doesn't make much sense for a programming language, it is thought that it was its original meaning.
Another, and now most commonly accepted meaning is PHP: Hypertext Preprocessor, a recursive or contrived acronym which is commonly used in OpenSource naming conventions.
It has also been thought to mean People Hate Perl by those who have embraced PHP.