Display Javascript variable into PHP variable

In my form action there's a url: www.url.com/?quantity=$quantity

And in the form there's a select box where customers choose the quantity.

<form method="post" name="jform" action="www.url.com/?quantity=$quantity">
<select class="font_12" id="quantity" name="quantity">
    <option value="10">10 PCs</option>
    <option value="25">25 PCs</option>
    <option value="50">50 PCs</option>
    <option value="99">99 PCs</option>
</select>

I am trying to get the value in the select box using ajax, and then display into the action form url. I did a alert and it works, I am getting the value of the select box. But I don't know how to put this vaue into the PHP varaible $quantity?

Here's my Ajax code:

$('#quantity').on('change', function() {
var val = $(this).val();
if(val != '') {
    $.get('index.php', {'quantity' : val}, function(resp) {
        alert(val);
    });
}
});

Actually I want it to change the php variable right away when the quantity in the select box change before submitting the form.

Any help?