I looked that up in SELFHTML which is in german (www.teamone.de/selfhtml) but a very good resource ...
javascript for loading a document into another frame:
parent.frames[1].location.href = "page2.php";
i dont think this is possible in php alone because of its concept: php is server-side and doesn't know anything about the frameset the user actually has in his client.
hope this was helpful
matto