i would use a switch case statement:
<?php
if(isset($_POST['select'])) {
switch($_POST['select']) {
case "1":
$redirPage = "page1.php";
break;
case "2":
$redirPage = "page2.php";
break;
case "3":
$redirPage = "page3.php";
break;
default:
$redirPage = "default.php";
break;
}
header("Location: ".$redirPage);
}
?>
this example assumes the name of the select is select, adjust accordingly.