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:
PHP Code:
if (isset($_POST['nav']))
{
switch ($_POST['nav'])
{
case "http://css-tricks.com/":
case "http://digwp.com/":
case "http://quotesondesign.com/":
header('Location: ' . $_POST['nav']);
break;
default:
// 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.