POO - SINGLETON CLASS

<?php
/**
 * Class Singleton is a generic implementation of the singleton design pattern.
 *
 * Extending this class allows to make a single instance easily accessible by
 * many other objects, using the {@link Singleton::getInstance()} method.
 *
 * @author     Quentin Berlemont <quentinberlemont@gmail.com>
 * @copyright  Copyright (C) 2010 Quentin Berlemont
 * @license    http://www.gnu.org/licenses/lgpl.html LGPLv3
 */
abstract class Singleton
{
    /**
     * A protected constructor; prevents direct creation of object.
     *
     * @param  void
     * @return void
     */
    protected function __construct()
    {
    }

    /**
     * Prevents users to clone the instance.
     *
     * @param  void
     * @return void
     * @throws LogicException If one tries to clone the instance.
     */
    final public function __clone()
    {
        throw new LogicException(sprintf('Cloning an instance of `%s` is not allowed.', get_class($this)));
    }

    /**
     * Returns a single instance of a class the static method is called in.
     *
     * See the {@link http://php.net/lsb Late Static Bindings} feature for more
     * information.
     *
     * @param  void
     * @return object Returns a single instance of a class.
     */
    final static public function getInstance()
    {
        static $instance = null;

        return $instance ?: $instance = new static;
    }
}

// Example
class Foo extends Singleton
{
    public function __toString()
    {
        return __CLASS__;
    }
}

class Bar extends Singleton
{
    public function __toString()
    {
        return __CLASS__;
    }
}

$foo = Foo::getInstance();

// prints: Foo
echo $foo;

$bar = Bar::getInstance();

// prints: Bar
echo $bar;

// prints: bool(true);
var_dump($foo === Foo::getInstance());

// prints: bool(true);
var_dump($bar === Bar::getInstance());

// prints: bool(false)
var_dump($foo === $bar);

// prints (LogicException): Cloning an instance of `Foo` is not allowed.
$baz = clone $foo;

?>