Select Statement Structure (Placeholders)

        $sql = "SELECT * FROM users WHERE username = ? LIMIT 1";
        $stmt = $pdo->prepare($sql);
        $stmt->execute([$username]);

       while ($rows = $stmt->fetch(PDO::FETCH_ASSOC)){

             $id            = $rows['id'];
             $uname         = $rows['username'];

             $_SESSION['id']            = $id;
             $_SESSION['user']          = $uname;
             header("Location: welcome.php");

       }