PDO将id号回显到SESSION中

这是我用来登录用户的代码，我想获得用户的id号，并将其回声到$_SESSION['Eacher']中以供以后使用，我想你可以使用$row['id']来完成，但似乎不是。。任何帮助都将不胜感激。

public function verifyLogin($username, $password){
    // this function checks whether the user name exists and if its a match
    if(!empty($username) && !empty($password)){
        $st = $this->db->prepare("SELECT * FROM teachers WHERE username=? AND password=?");
        $st->bindParam(1, $username);
        $st->bindParam(2, $password);
        $st->execute();
        if ($st->rowCount() == 1){
            session_start();
            $_SESSION['Teacher'] = $row['id'];
            header("Location: dashboard.php");
        } else {
            header("Location: login.php?incorrect");
        }
    } 
}