Wordpress add_submenu_page won't use class object?

I am trying to make my Wordpress submenu page a function from within a class that extends the parent class. The parent class expects Object

as a parameter in it constructor

.

Object got in parent's constructor and so far seems fine, but when the class later runs the page_form method $this->model

it is null and I get Call to a member function get_primary_key() on a non-object

.. Why?

I am passing a class reference to wordpress add_submenu_page

, is this something I am doing wrong in this code below? Or am I structuring my code in some way?

I want to have a class or method that displays a form that other classes can use and pass a unique object to that form. I don't need help creating the form or objects, but I need some guidance on how to do this inheritance, preferably object oriented.

My parent class:

<?php
class Backend
{
    protected $model;

    function __construct($model)
    {
        $this->model = model;
        echo $this->model->get_primary_key() // this works fine. returns 'id' as string.
    }

    function page_form()
    {
        echo $this->model->get_primary_key(); // this gives me error,  Call to a member function get_primary_key() on a non-object.
        // This function should render a form, 
        // using parameters inside $this->model, but its NULL.
    }

}

      

        
        
        
      

    

My main plugin file:

<?php

// ...

if(is_admin()) {
    add_action( 'admin_menu', 'my_plugin_menu' );
}

function my_plugin_menu() {
    add_menu_page(
        'My plugin', // page-title
        'My plugin', // label
        'manage_options', 
        'my-plugin-menu', // unique-handle
        'settings_start' // function
    );

    $bookings = new Bookings();
    add_submenu_page( 
        'my-plugin-menu', // parent unique-handle
        'Add new Booking', // page-title
        'Add new Booking', // label
        'manage_options', 
        'my-plugin-menu-add-booking', // submenu unique-handle
        array(&$bookings, 'page_form') // this is the function to call, defined in parent class.
    );

?>

      

        
        
        
      

    

My booking class:

<?php 

use WordPress\ORM\Model\BookingModel;
class Bookings extends Backend
{
    function __construct()
    {
        parent::__construct(new BookingModel());
    } 

    // ...

}

?>