Back button with method=post parameter

I’ve been looking an answer to this question in the forums and google, but I haven’t found anything.
Sorry if it’s a stupid question, but I need to do this. Maybe it won’t be the correct way.

I have a post, and I’m trying to create a button in my post to go back to the index page.
The index page shows all the posts classified by categories in a tabbed section. So, when the user clicks in the back button, it should go to the index page, but the correct tab must be selected.
So, the way to do this for me, is to create a form with a submit button and a hidden parameter containing the post category.
I’m trying to recover the hidden in the índex page:

$back = $_POST['hidden_name'];

But it doesn’t work. If i change the method to GET in the form and recover it with $_GET, it works.
Looks like some redirection transforming the POST to GET.

Someone could tell me what’s i’m doing wrong?

Thank You.