class Admin extends User
{
    // Other methods

    public function save()
    {
        parent::save();
        // now save your additional fields wherever they need to go
    }
}

$entity = new Admin;
$mapper = new AdminMapper( $db );

$entity->setName('Wintermute');
$mapper->fetch( $entity );
//retrieve data for admin with that name

$entity->setName('Neuromancer');
$mapper->store( $entity );
// rename and save

class User
{
  public function Save()
  {
    $savestuff = $this->GetProperties();
    // do save acctions
  }

  protected function GetProperties()
  {
    return 'user prop';
  }
}

class Admin Extends User
{
  protected function GetProperties()
  {
    return  'admin prop';
  }
}

class Admin
{
    public function save(Model_Abstract $model = null)//type hinting
    {
        if ($model instanceof Model_Admin)
        {
            //do admin insert/update stuff here
            return;
        }
        parent::save($model);//call regular method
    }
}

class BasicEntity
{

    protected $tablename;

    protected $schema;

    public function update()
    {

        $fields = "";
        $placeholders = "";

        foreach($this -> schema as $field => $type)
        {

            // you join the fields here to get something like ('username', 'email', 'enabled', 'createdAt', 'password')
            // then you write your PDO statement providing placeholders like (:?, :?, :?, :?, :?)
            // you'll have to bind parameters based on their $type [int, string, date]

        }

        $query = sprintf(
            "UPDATE %s SET VALUES(%s) = %s",
            $this -> tablename,
            $fields,
            $placeholders
        );

        // execute statement here, handle exceptions, and so...

    }

}

class User extends BasicEntity
{

    protected $id;
    protected $username;
    protected $email;
    protected $password;
    protected $enabled;
    protected $createdAt;

    public function __construct()
    {

        $this -> tablename = '_user';
        $this -> schema = array(
            'id'        => 'int',
            'username'  => 'string',
            'email'     => 'string',
            'password'  => 'string',
            'enabled'   => 'int',
            'createdAt' => 'datetime'
        );

    }

}

class Admin extends User
{

    protected $additionalProperty;

    public function __construct()
    {

        parent::__construct();

        $this -> schema['additionalProperty'] = 'string';

    }

}