The only way to do this is via AJAX. I'm not sure you understand the difference between server side processing and client side processing. Javascript is client side processing - it happens once the code has reached the user's browser. The only way it can call PHP, which is on the server, is by submitting a form or making a request back to the server. In the days before AJAX I had a technique for fudging it but nowadays you are much better off choosing an AJAX library (such as prototype, but there are others) and learning how to use it.
I'd suggest jQuery as its well documented, you'll find some good examples to get your started if you search in the right places.