system/classes/singleton.php

<?php
namespace Habari;

abstract class Singleton
{
  // Single instance of class available.
  private static $instances = array();

  protected static function instance()
  {
    $class = get_called_class();
    return self::getInstanceOf($class);
  }

  /*
   * The overridden methods can't have a different signature, so there needs
   * to be two functions: one, internal to a class, that calls this one
   * (a singleton factory) that produces objects of the requested classes
   */
  protected static function getInstanceOf( $class )
  {
    if ( ! isset( self::$instances[$class] ) ) {
      self::$instances[$class] = new $class();
    }
    return self::$instances[$class];
  }

  protected final function __construct() {}
  private final function __clone() {}
}

?>