You appear to be endorsing a very poor practice by example.

At the very least remove the onchange behaviour and get that “go” button out of the noscript.
That’d make it at least usable to most people.

$dropdown_nav = array("http://www.google.com","http://www.css-tricks.com","http://www.yahoo.com");

if ($_GET['page_nav'] && in_array($_GET['page_nav'],$dropdown_nav)) {
	header("Location:".$_GET['page_nav']);
        exit();
}

// Loop through the array as an option to select form here

This is a way to combine the obtrusive method with the PHP stuff to make it work the same way, only without jQuery:

<form action="" method="post">
    <select name="nav" onchange="window.open(this.options[this.selectedIndex].value)">
        <option value="">Go to page...</option>
        <option value="http://css-tricks.com/">CSS-Tricks</option&gt;
        <option value="http://digwp.com/">Digging Into WordPress</option>
        <option value="http://quotesondesign.com/">Quotes on Design</option>
    </select>
    <noscript>
        <input type="submit" value="Go" id="submit" />
    </noscript>
</form>

<?php
        if (isset($_POST['nav'])) {
                // you should validate $_POST[nav] here
                header("Location: $_POST[nav]");
        }
?>