One way to break out of a frameset is to use a 'target' of
'_top' in the href, like this:
<a href='nextpage.php4' target='_top'>Next Page</a>
This will clear all frames and load nextpage.php4 in the current window.
Breaking out of a frameset is client side, so I don't think you can use a url or header to change the frameset without reloading the main frame url, which is in window '_top'.
At least I've never seen a way to use the URL or header (with the exception of the 'target' directive and/or javascript) to change which window/frame is updated.
Hope this helps.
-Rich