devi mettere $username in $_SESSION se vuoi passarlo tra le pagine ...

codice:
....

$_SESSION['SESS_MEMBER_ID'] = $member['iduser'];
$_SESSION['username'] = $username;


.......

   if(empty($_SESSION['SESS_MEMBER_ID']) ) {
     include ('login.php');
    } else {

        echo "Ciao $_SESSION['username']";
      }