// Variable "id" is set?
if (isset($_GET['id'])) {

    // Set variable "id"
    $id = $_GET['id'];

    // Create whitelist
    $whitelist = array(1, 3, 5);

    // Check if element is in array
    if (in_array($id, $whitelist)) {
        // Do something...
    } else {
        header("Location: http://xyz.de/errorpage");
        die();
    }
}