Using header(0 to manage user inactivity

I'd like to be able to "timeout" a user if they are inactive for X minutes. Say, if they walk away from the browser leaving sensivive data in plain view. Is there any reason that using the header() function as below wouldn't be a reliable way to do this (albeit user-unfriendly)? It "works for me" on my Mac with various browsers, but I wonder how universally it will work. $timeout = 900; // 15 minutes header('Refresh: ' . $timeout . ';url=www.whatever.com/logout.php');