В сценарии ваша форма отправляется:
if ($_POST['dropdown-name'] == 'Page 3') {
header("Location: page3.php");
} else if ($_POST['dropdown-name'] == 'Page 4') {
header("Location: page4.php");
} // ... etc
~
$pages = array('Page 1' => 'page1.php', 'Page 2' => 'page2.php', 'Page 3' => 'page3.php');
if (array_key_exists($_POST['dropdown-name'], $pages)) {
header("Location: " . $pages[$_POST['dropdown-name']]);
} else {
echo "Error processing form"; // submitted form value wasn't in your array, perhaps a hack attempt
}