How: Javascript send variables to PHP without sitechange

Is it possible to send Variables from Javascript in a HTML-file to a PHP file without changing the current page or opening a new Window?

I know there’s a way in flash, but I cannot imagine a way in JS

I.e.:
A Shop site where you click on an item, Javascript tells a PHP file to add this Item to the User’s Cart, without doing anything to the current site or opening a popup