Magento: How can I get at all the cart data from the form_key

I have added a form onto the bottom of the cart page, and changed the cart page to redirect to my own custom page.

This custom page currently just outputs the submitted data from the previous page, like so: print_r($_POST);

This produces the following output:

Array
(
    [form_key] => 2AJOmd7GhFVICntG
    [cart] => Array
        (
            [20] => Array
                (
                    [qty] => 1
                )
        )
)

So my question is, how can I get at all the cart data from the form_key.

A solution in pure PHP would be preferable. This is Magento 1.9.x.