Post (method="post") the html control data e.g. txtusername, txtpassword to your php page. Then on your php page you need to start_session() (or something like that) and then assign a value to the session variable e.g. $_SESSION['varUserName'] = $_POST[txtusername].
Now you can access that variable while the session is live.
e.g.
<?php
echo $_SESSION['varUserName'] //output user name
?>
My php is a bit rusty so it might not be exact.