Example PHP singleton class for reference. For the record, singletons are evil: http://stackoverflow.com/a/138012/27025

Singleton.php

<?php

    /**
     * Example PHP singleton class for reference.
     * For the record, singletons are evil:
     * http://stackoverflow.com/a/138012/27025
     */
    class Singleton {

        /**
         * Private contructor to prevent direct access
         */
        private function __construct() {
            // NULL
        }

        /**
         * Return an instance of the class
         * @return object Instance of Singelton()
         */
        public static function Initialize() {

            // Setting some defaults
            static $instance = null;

            // Create new object if none exist
            if ($instance === null) {
                $instance = new Singleton();
            }

            // Return object
            return $instance;

        }

    }

    $foo = Singleton::Initialize(); // Returns Singleton object
    $bar = new Singleton();         // Trows error