Creating a comments system with showing authors

Question

I want to create a simple comments system with PHP. I have a registration system so only logged users can add comment. I have a row in my table comments called idauthor. I using this sql query to add a new comment:

$sql = 'INSERT INTO kawaly SET
        tekst="' . $tekst . '",
        autor="' . $autor . '",
        datapasty=NOW(),
        tytul="' . $tytul . '",
        idauthor="' .$id.'"';

my variable $id in this query cames from my logging system:

$ilu_userow = $rezultat->num_rows;
            if($ilu_userow>0)
            {
                $_SESSION['zalogowany'] = true;

                $wiersz = $rezultat->fetch_assoc();
                $_SESSION['id_user'] = $wiersz['id'];

Okey then. When i adds a new comment, i can see in row idauthor a correct value. Now i can show it by simple relation query using INNER JOIN user ON user.id = idauthor. But when i have two people logged in, it`s does not sending correctly value for idauthor. How can i modify it on correct way? How can i insert into my database ID of user which actually wrote a comment for a proper method? Thank you for any tips! Regards.