<?php
/**
* 
*/
abstract class Singleton
{
  static private $__object_pool = array();
  static protected $__use_args_sig = false;
  /**
  * 
  */
  private function __construct() {/*...*/}
  /**
  * 
  */
  private function __clone() {/*...*/}
  /**
  * 
  */
  protected function __init() {/*...*/}
  /**
  * 
  */
  static final function get_instance()
  {
    $args = func_get_args();
    $class = get_called_class();
    // By default we use class name as a key.
    $key = $class;
    if (static::__use_args_sig() && count($args)) {
      $sig = md5(join($args));
      $key .= $sig;
    }
    if (!isset(self::$__object_pool[$key])) {
      $instance = new $class();
      call_user_func_array(array($instance, '__init'), $args);
      self::$__object_pool[$key] = $instance;
      return $instance;
    }
    else {
      return self::$__object_pool[$key];
    }
  }
  /**
  * 
  */
  static final function __singleton_add_instance($instance)
  {
    $class = get_class($instance);
    if (array_key_exists($class, self::$__object_pool)) {
      return;
    }
    self::$__object_pool[$class] = $instance;
  }
  /**
  * 
  */
  static protected function __use_args_sig()
  {
    return false;
  }
}

//
// Implementation of DB class.
//
class DB extends Singleton
{
  function __init()
  {
  }
}

//
// How to use
//

// Connection to default database.
$db = DB::get_instance();

// Execute query
$db->select('select * from `test_tab`);

// Send query to another database where $dsn is a database source name.
$db2 = DB::get_instance($dsn);
$db2->select('select * from `test_tab`);
?>