What is PHP?
PHP is a server-side, cross-platform, HTML embedded scripting language. If you are completely new to PHP and want to get some idea of how it works, have a look at the Introductory Tutorial. Once you get beyond that have a look at the example archive sites and some of the other resources available in the Links section.
So, how much does it cost?
This may sound a little foreign to all you folks coming from a non-Unix background, but PHP doesn't cost anything. You can use it for commercial and/or non-commercial use all you want. You can give it to your friends, print it out and hang it on your wall or eat it for lunch. Welcome to the world of Open Source software! Smile, be happy, the world is good. For the full legalese, see the official license.
Year 2000 Compliance
Just like your C compiler or your favourite text editor, PHP is just about as year 2000 compliant as your pencil. If you're still worried (perhaps all pencils are built to break on December 31st 1999?)