How to use AJAX to update Shopping Cart Item Count

WPEC 3.8.9.5
Gold Cart 2.9.7

I know, I’m missing probably very basics of PHP and AJAX and this might be a lame question. Never mind, I’ve got following JS/jQuery code to invoke an AJAX when visitor clicks any of the Add to Cart buttons

// Update the number by shopping cart icon when item is added to the cart
function itemCount() {
$.ajax( {
url : "cart-count.php",
type : "POST",
data : "id=1",
success : function(msg) {
alert (msg); //or use data string to show something else
}
});
}

$(document).on('click', 'input.wpsc_buy_button', itemCount );

I also have following PHP code

$cart_count = wpsc_cart_item_count();

How can I call the PHP code via AJAX and store its result in a JS variable, so I can place it somewhere in front-end of header of my website?

http://wordpress.org/extend/plugins/wp-e-commerce/