Simple Basic Singleton

App.class.inc

class App
{
  public static $instance = null;

  public $instance_id = null;
  public $db;

  function __construct()
  {
  }

  public static function getInstance()
  {
    if(self::$instance==null)
    {
      self::$instance = new App;
      self::$instance->instance_id = time();
      self::$instance->db = DB::getInstance();
    }
    return self::$instance;
  }
}

DB.class.inc

class DB
{
  public static $instance = null;

  public $instance_id = null;

  function __construct()
  {
  }

  public static function getInstance()
  {
    if(self::$instance==null)
    {
      self::$instance = new DB;
      self::$instance->instance_id = time();
    }
    return self::$instance;
  }
}

loader.php

<?php
require_once 'App.class.inc';
require_once 'DB.class.inc';

$app = App::getInstance();
$app2 = new App();
$db = DB::getInstance();
?>
<pre><?php var_dump($app) ?></pre>
<pre><?php var_dump($db) ?></pre>
<pre><?php var_dump($app2) ?></pre>