<?php
class Singleton {
    static private $instance = false;
    private function __construct() {}
    private function __clone() {}
    static function instance () {
        if (self::$instance === false) {
            self::$instance = new Singleton;
        }
        return self::$instance;
    }
}
$obj	= Singleton::instance();
var_dump($obj);
$obj	= null;
var_dump($obj);
$obj = Singleton::instance();
var_dump($obj);

object(Singleton)#1 (0) {
}
NULL
object(Singleton)#1 (0) {
}