I've seen this question asked 4 times today, and answered it three of them... :p
The problem is that as a default, global variables is now OFF. You need to reference it like this, if you are using POST with your form:
$_POST["username"];
or this, if you are using GET
$_GET["username"];
and, seeing how you are using neither, you will also need to set that:
<form action="<?PHP echo $_SERVER["PHP_SELF"]; ?>" method="post">