I'd suggest a switch though. PHP is a string based language so it is primitive which means you can switch on the string. It simply allows for easier additions should you choose in the future:
header('Location: ' . $_POST['nav']);
// in case you want to do something for every other condition.
exit() or die() should be called right after the header unless you want the remaining script to still continue processing. Issuing a header doesn't terminate the script run, it simply tells the browser to redirect to a new location.