logout.php:
<?php
session_start();
$_SESSION = array();
session_destroy();
header("Location: /login.php");
exit;
?>

login.php:
<?php
session_start();
if(isset($_SESSION['auth'])) {
  header("Location: /hemmelig.php");
  exit;
} elseif(isset($_POST['username']) && isset($_POST['password'])) {
  if($_POST['username'] == 'knutola' && $_POST[''] == 'mittpassord') {
    $_SESSION['auth'] = true;
    header("Location: /hemmelig.php");
    exit;
  } else {
    echo "Feil brukernavn eller passord";
  }
} else {
  echo '<form method="POST" action="/login.php">Username: <input type="text" name="username" /><br/>Password: <input type="password" name="password" /><br/><input type="submit" value"Login" /></form>';
}
?>

hemmelig.php:
<?php
session_start();
echo 'Her er det hemmelige innholdet<br/><a href="/logout.php">Logg ut</a>';
?>