PHP stands for PHP Hypertext Preprocessor
PHP use to stand for Personal Home Page, it was a library a guy wrote to be able to build his homepage with better embedded functionality and it expanded from there..
If you dont know what it is, I wouldnt recommend you try to figure it out unless you are a programming/web enthusiast.
You can get it from
http://www.php.net
You need a webserver (Apache) to go along with it to use this stuff we're posting..