Demonstration of the Singleton design pattern in PHP.

Singleton.php

<?php
namespace FlorianWolters\Example\Pattern\Design\Creational\Singleton;

/**
 * Demonstration of the *Singleton* design pattern in PHP.
 *
 * This is the correct implementation of the *Singleton* creational design
 * pattern in the PHP programming language.
 *
 * - Prevents cloning of an instance via the `__clone()` magic method.
 * - Prevents unserializing of an instance via the `__wakeup()` magic method.
 *
 * @author    Florian Wolters <wolters.fl@gmail.com>
 * @copyright 2011-2012 Florian Wolters
 * @license   http://gnu.org/licenses/lgpl.txt LGPL-3.0+
 * @link      http://gist.github.com/3664366
 * @since     Class available since Release 0.1.0
 */
final class Singleton
{
    // Declaring the class as "final" is required since one subclass would lead
    // to unexpected behaviour.

    /**
     * Returns the *Singleton* instance of this class.
     *
     * @return SingletonInterface The *Singleton* instance.
     */
    public static function getInstance()
    {
        // The *Singleton* instance of this class.
        static $instance = null;

        if (null === $instance) {
            $instance = new self;
        }

        return $instance;
    }

    // @codeCoverageIgnoreStart

    /**
     * Protected constructor to prevent creating a new instance of this class
     * via the `new` operator.
     */
    protected function __construct()
    {
        // The constructor could also be declared with the visibility "private".
    }

    /**
     * Private clone method to prevent cloning of the instance of this class.
     *
     * @return void
     */
    final private function __clone()
    {
    }

    /**
     * Private unserialize method to prevent unserializing of the instance of
     * this class.
     *
     * @return void
     */
    final private function __wakeup()
    {
    }

    // @codeCoverageIgnoreEnd
}