使用PDO（php）不在子类中映射SuperClass字段

I have the PersistentObject class as following

abstract class PersistentObject implements IPersistable
{
    private $id;

    public function getId()
    {
        return $this->id;
    }

    public function setId($id)
    {
        $this->id = $id;
    }
}

and the UserModel extending the PersistentObject

class UserModel extends PersistentObject
{
    public static $TABLE_NAME = "user";

    private $email;

    private $username;

    private $password;

    public function getEmail()
    {
        return $this->email;
    }

    public function setEmail($email)
    {
        $this->email = $email;
    }

    public function getUsername()
    {
        return $this->username;
    }

    public function setUsername($username)
    {
        $this->username = $username;
    }

    public function getPassword()
    {
        return $this->password;
    }

    public function setPassword($password)
    {
        $this->password = $password;
    }
}

now when I fetch the usermodel with pdo

$entity = $stmt->fetchObject("UserModel");

I'm getting the result ( var_dump($entity) ):

object(UserModel)[11]
  private 'email' => string 'andrewwww@gmail.com' (length=24)
  private 'username' => string 'andrewww' (length=13)
  private 'password' => string '72bed4064dbe53d7fc5fd078214387c813c1f670' (length=40)
  private 'id' (PersistentObject) => null
  public 'id' => string '2' (length=1)

and if I try

var_dump($entity->getId());

I receive null;

How is it possible to map the superclass fields into the subclass??? thnx!

That's exactly the difference between private and protected.

From http://php.net/manual/en/language.oop5.visibility.php:

Members declared protected can be accessed only within the class itself and by inherited and parent classes. Members declared as private may only be accessed by the class that defines the member.

So use protected in the parent class:

abstract class PersistentObject implements IPersistable
{
    protected $id;
    ...