Problem with http basic authentication

I am having trouble getting basic authentication, so I wrote this simple file which I was hoping would reveal why.

     1  <?php
     2
     3  if (!isset($_SERVER['PHP_AUTH_USER'])) {
     4    header ('WWW-Authenticate: Basic realm="Basic Auth Test"');
     5    header ('HTTP/1.0 401 Unauthorized');
     6    die ("Was the Cancel button pressed?");
     7  }//end if
     8
     9  echo "\nOk, we have a value! Let's dump the \$_SERVER global array!\n\n";
    10  var_dump($_SERVER);
    11  echo "\n\nFinished!\n";
    12
    13  ?>
    14

It should open an 'Authentication Required' window requesting a user name and password, but should accept whatever is entered. However, the window appears as expected, but when the fields are filled and OK is clicked, it vanishes for a very brief time, then reappears. Seems the entries are not being kept in the $_SERVER['PHP_AUTH_USER'] array.

The server is running Centos 6.5 and Plesk 12

I tried running the script on another server with Centos 6.4 with no GUI, and it works fine when I browse it.

Kind of making me crazy. Hope someone can help.

Wayne