Hi, i am having a few problems i have only just started php

but could you take a look at this and tell me why its not pulling the $username from the database

Many Thanks

PHP Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<?PHP
$username = $HTTP_SESSION_VARS['username'];
?>
<body>
<h1>Well Done</h1><br>
<?php echo"$username" ?> is <font color="#FF0000">logged in</font>
</body>
</html>
Thanks again