PHP's fun. It's a programming language, unlike XHTML (a markup language). So it lets you use things like conditional logic (if statements), loops, variables, etc. that you'd never use or see in HTML. In your typical use case, PHP scripts are run server-side and output X/HTML markup, which is then sent to the browser just like any other web page.
That's an incredibly brief overview of what it does, but the bottom line is PHP and HTML are very different beasts. PHP can be incredibly useful, provided you have a use for it, but it's not going to make your web pages look sleeker (just more functional).
If you'd like a bit more, you can check out W3Schools--from what I've heard their PHP tutorials are pretty good:
http://www.w3schools.com/PHP/php_intro.asp
Minor caveat: although PHP is fun (IMO) and pretty beginner-friendly, it's not necessarily the best first language for programmers. This is really because some of the things it does to be more accessible to beginners also tends to teach lazy principles of programming that don't carry over well to other areas and are best never learned. So if you're serious about learning to program, I'd honestly consider playing around with C or something before PHP. It's not impossible to go the other way around--I started with PHP myself--but I think it's easier to start with something like C first.