Header: Opening URL in New Window

Here’s what I would like to do - open a new window with one URL and update the current window with another:

    if (@mail($to, $subject, $body, $header))

    header('Location: http://www.mydomain.com/thanks.html');
    header('Location: http://www.myshoppingcart.com/category.sc?categoryId=2; Window-target: _blank');


Doesn’t work, of course. The second header is the one that works, but it simply updates the current window (no new window opens).

Thanks for any help.