Understanding session variables with input field and register_global

I've an input field in a form <input name="username" type="text" ... and with register_global I can use this field in PHP as variable $username. Yet if I use a session variable $_SESSION['username'] = 'value' the variable $username gets the same value. On the other side when I enter a value in the input field, the session variable isn't changed. So how can I set the session variable from the input field after it has changed? O. Wyss