CPanel PHP_AUTH_USER behaving unexpectedly

I am ruining the following code:

<?php
if (!isset($_SERVER['PHP_AUTH_USER'])) {
    header('WWW-Authenticate: Basic realm="My Realm"');
    header('HTTP/1.0 401 Unauthorized');
    echo 'Text to send if user hits Cancel button';
    exit;
} else {
    echo "<p>Hello {$_SERVER['PHP_AUTH_USER']}.</p>";
    echo "<p>You entered {$_SERVER['PHP_AUTH_PW']} as your password.</p>";
}
?>

Straight from the PHP website. However, When I run this script on a CPanel server it just keeps prompting me for my login details. Wh开发者_运维知识库ereas it works fine on a server not running CPanel.

Any ideas? Obviously a configuration issue here.

Thanks