PHP will allow you to program your web pages and to accomplish things that with regular HTML you would not be able to do...
Take a look here:
http://www.php.net/[
^] where you will be able to find whatever you could need regarding PHP.
If you want to test PHP there are plenty of suites out there that will allow you to run a php server in your computer or in a virtual machine... I'm using
EasyPHP[
^], but there are many others...
Good luck!