header Authenticate problem

Har følgende code:

function auth()
{
        // if empty, send header causing dialog box to appear
        header('WWW-Authenticate: Basic realm="Husk at din email er dit brugernavn"');
        header('HTTP/1.0 401 Unauthorized');
        echo "Error";

        exit;
}
   
    if(isset($_SERVER['PHP_AUTH_USER']))
        $user = $PHP_AUTH_USER;
    else
        $user = "";

    if(!isset($_SERVER['PHP_AUTH_USER']))
        $pass = crypt(md5($_SERVER['PHP_AUTH_PW']), md5($_SERVER['PHP_AUTH_PW']));
    else
        $pass = crypt(md5($_SERVER['PHP_AUTH_PW']), md5($_SERVER['PHP_AUTH_PW']));

   

      $sql = "SELECT * FROM user WHERE email='$user' AND pass='$pass'";

    $result = mysql_query($sql) or die(mysql_error());

   
    $row = mysql_fetch_array($result);
    $user_id = $row['id'];

   
        // check if the username and password are correct       
        if ($user_id)
        {
            // dispay happy message and admin stuff
    echo 'Done';
           
        }
        else
        {
            // display angry message for invalid user

    echo auth();;
        }


og kommer med følgende fejl:

Notice: Undefined index: PHP_AUTH_PW in auth.php on line 20

Notice: Undefined index: PHP_AUTH_PW in auth.php on line 20

Hvad er der galt?