rederencing other sub classes inside class

[Sheldon]

Hi,

I am making a simple PHP framework, this has a 'base' class, then includes other classes, like a database abstraction class and a form validation class.

I have got the classes all working and including, but I cant seam to reference another sub class from with in a sub class?

example:

<?php

    require_once('db.php'); // databse class
    require_once('validation.php'); // validation class
    
    class Base {
        
        function Base() {
            
            // load extending classes
            $this->db = new Db();
            $this->validation = new Validation();
            
        }
        
        function test() {
            
            print_r($this->validation->run());
            
        }
    }
    
    class Db extends base {
        
        function Db() {
            
        }
        
        function query($sql) {
            //
        }
    }
    
    class Validation extends base {
        
        function Validation() {
            
        }
        
        function run() {
            
            $this->db->query("SELECT * FROM `table`");
            
        }
    }

What I want to be able to do, is call a sub function from another sub function.

for example:

<?php

// load main class, and my two sub classes
$instance = new Base();

// fire a function which has access to my validation class, and my DB class.
$instance->test();

however I get the PHP Error

Fatal error: Call to a member function query() on a non-object

I have tried using the parent:: tag at the top of my subclasses like below, but that didn't seam to help;

    class Db extends base {
        
        function Db() {
            parent::System();
        }