Help needed... FORMS & PHP (not as noob as you think)

[Deaths]

Ok, here's my problem:

I'm currently using an XHTML form (duh), which sends some data to a PHP file.

Now, one of the text input boxes, looks like this:

<input name="products[$order[productid]][quantity]" type="text" class="bginput" value="1" size="2">

Lets say $order[productid] is 1, so the name will be this:

products[1][quantity]

Now, I was wondering how I can let PHP read that "1" (not static, but dynamic I mean. If it would be 2, it would read "2", if it would be "3" it would read "3".
I'm pretty sure this is possible, but I can't figure it out.

Help is appreciated, as always.