HTTP Basic Header Authentication with PHP

<?php

$valid_passwords = array ("username_one" => "password_one", "username_two" => "password_two");
$valid_users = array_keys($valid_passwords);

$user = $_SERVER['PHP_AUTH_USER'];
$pass = $_SERVER['PHP_AUTH_PW'];

$validated = (in_array($user, $valid_users)) && ($pass == $valid_passwords[$user]);

if (!$validated) {
 header('WWW-Authenticate: Basic realm="My Realm"');
 header('HTTP/1.0 401 Unauthorized');
 die ("Not authorized");
}