PHP is used to integrate data within databases into your content. So it can be used with other languages like HTML and Javascript.
One of the leading languages I think in the industry and a must have for web site development these days. I see that you might want to learn it so I suggest to you:
myhosting.com/forums Under their Programming Section and Beginners Section, you will see guides and tutorials for PHP. You can also ask questions if you get lost.
__________________ My Site: MacLappy [link coming soon!]
PHP is a server side language. It is executed on server and only the result is viewed by user. Javascript on the other hand is a client side language. You can compare this to in performance terms.