[felgall]

So the field is only allowed to have three specific values - to sanitize it you check that the value is one of those three -

<?php
	if (isset($_POST['nav'])) {
               if ($_POST['nav'] == "http://css-tricks.com/" ||
                   $_POST['nav'] == "http://digwp.com/" ||
                   $_POST['nav'] == "http://quotesondesign.com/")
		 header("Location: $_POST[nav]");
	}
?>