[PHP] Sessions error

[Mike Tuller]

I am working with sessions, and following an example in a book and am getting an error that I am not sure of. I have an html doc that has the fields for username and password and the information entered goes to this script.

<?
session_start();

// set variables needed
$username = $_POST['username'];
$password = $_POST['password'];

$connection = mysql_connect( 'localhost', 'username', 'password' ) or die (mysql_error());
$db = mysql_select_db( "database", $connection) or die (mysql_error());


$query = "select * from users where username='$username' and password=('$password')";
$result = mysql_query($query);
$num = mysql_num_rows($result);

if ($num > 0 )
{
$valid_user = $_POST[username];
session_register("valid_user");
}

// PHP script to create the html for the login page
if (session_is_registered("valid_user"))
{
echo "You are logged in as: $valid_user <br>";
echo "<a href=\"logout.php\">Log out</a><br>";
}
else
{
if (isset($username))
{
// if they have tried and failed to login
echo "Could not login with the username $username.";
}
else
{
// They have not tried to login yet or have logged out
echo "You are not logged in. Please click on the link below to go back to the login form.<br>";
}
}
?>
<br>
<a href="login.html">Back to Login Form</a>
</body>
</html>

Everything works except for when I login correctly, it shows that I am logged in, but at the bottom I get this message.

Warning : Unknown(): Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively. in Unknown on line 0

What does this mean, and what am I doing wrong?

Thanks,
Mike