Basic Authentication fails to authenicate

[PeDa]

I am trying to protect a webpage with basic autentication, however (both on a local WAMP-server and on my providers server), I cannot get any authentication. Even with the following code which should, I assume, only check for the presence of any username, authentication is refused:

<?php
  if (!isset($_SERVER['PHP_AUTH_USER'])   ) {
    header('HTTP/1.1 401 Unauthorized');
    header('[URL unfurl="true"]WWW-Authenticate:[/URL] Basic realm="Bla"');
    exit('Must enter valid username and password to continue.');
  }
?>

This derived from the following (described as working):

<?php
  // User name and password for authentication
  $username = 'rock';
  $password = 'roll';

  if (!isset($_SERVER['PHP_AUTH_USER']) || !isset($_SERVER['PHP_AUTH_PW']) ||
    ($_SERVER['PHP_AUTH_USER'] != $username) || ($_SERVER['PHP_AUTH_PW'] != $password)) {
    // The user name/password are incorrect so send the authentication headers
    header('HTTP/1.1 401 Unauthorized');
    header('[URL unfurl="true"]WWW-Authenticate:[/URL] Basic realm="Guitar Wars"');
    exit('<h2>Guitar Wars</h2>Sorry, you must enter a valid user name and password to access this page.');
  }
?>