Gathering Userinput For A Small Calculator?

What i am trying to do is the following: I read a tutorial on objects and thought i would try it out. The tutorial taught me how to make a function calculating the areal and circumspherence of the circle where the user input would be the radius.

To sum up: I want to make it possible for the user to write, say, "2" in the form text field and click "Calculate" and then the text will appear w/o reloading the page. I could easily do this by removing the objects but as i am training my object skills i would like to know if it is possible to do this with objects?

The problem is that i dont know how to take the user input. I know that in PHP it is "$_POST["name"]" but what is it in JS? And is it possible to jump into PHP and use $_POST inside the JS script or how can this be done?

This is my code so far (It prolly looks strange but i really dont know how to get it to work).

<script type="text/javascript">
function Circle(radius){
this.radius = radius;
this.getArea = function(){

[Code]....

As you can see i have a predefined object: bigCircle. What i want to do is change the new Circle(100) to new Circle(USER INPUT HERE).