Получить имя по id

function username_from_id($from_username){
    $query = ("SELECT `from` FROM `messages` WHERE `from` = '$from_username'");
    $username_id = $query['from'];
    $row = ("SELECT `username` FROM `users` WHERE `user_id` = '$username_id'");
}

$from_username = $messages['from'];
echo username_from_id($from_username);//не выводит ничего

$query = "SELECT users.username
            FROM users AS users, messages AS messages
           WHERE users.user_id = messages.from
             AND messages.from = '$from_username';";

$from = $user_data['user_id'];
                            $to = $_GET['to'];
                            $query = mysql_query("SELECT * FROM `messages` WHERE `from` = $from AND `to` = $to OR `from` = $to AND `to` = $from ORDER BY `id` DESC") or die(mysql_error());
                            $messages = mysql_fetch_array($query);

$query = ("SELECT * FROM `messages` WHERE `from` = '$from_username'") or die(mysql_error());

$query = mysql_query("SELECT * FROM `users` WHERE `user_id` = '$from_username'") or die(mysql_error());
    echo $query['username'];