Im a little confused about this. I've only used php and html till now and wish to make my site web2.0-ish like most ajax based sites are. Are there any libraries i can use based on php? I know php is server side.. is there a client side ajax library or something?
"is there a client side ajax library or something"
-That's what jQuery is. Look at the .get() and .post() functions in jQuery. They'll come in handy.
On the server side, in PHP, you treat it like a normal page load.
Server Side: PHP
Client Side: HTML for markup (static content once it's served up by your PHP script) and JavaScript for scripting (making things happen dynamically in the user's browser)
Things like JQuery are libraries (usually called frameworks in the Javascript world) built in JavaScript. There are a number of frameworks out there. JQuery is a big one, Dojo is another popular one. Have a look at http://en.wikipedia.org/wiki/Comparison_of_JavaScript_frameworks to get a basic feel for the major players.
My suggestion: Spend a few hours writing a basic JavaScript tool following something like http://www.w3schools.com/js/default.asp When you have an understanding of what Javascript is capable of, start playing with one a framework.
It's not the most documented tool out there, but one of the easier ways for a PHP person to put AJAX into your application is to look toward XAJAX (http://www.xajaxproject.com. In a nutshell, you install XAJAX, pass calls through it to PHP functions, and it does the transaction asynchronously like AJAX, without having to write a ton of Javascript. Be aware that you're not going to get completely away from Javascript, and there are many other solutions out there (such as Jquery, which is a great choice for many Javascript needs) My only real complaint is that much of the XAJAX community is based in Europe, so the forums (in English) can get a bit garbled in translation.
My company uses a combination of the two partially because XAJAX feels more "comfortable" to php-only developers. When we need something that's more robust, I nearly always go the Jquery route.