If you're doing PHP header redirect, the name of the page has to be given as a full url, complete with the [url]http://[/url] and everything. To avoid problems if you try to run your script on a different server, you can do this:
header("Location: [url]http://[/url]".$SERVER['HTTP_HOST'].dirname($SERVER['SCRIPT_NAME'])."/whatever.html");
That would redirect to the page "whatever.html", in the same directory as the script you called the function from.