객체 단일형성에 직면하다

<?php

	class A{
		private static $ob=null;
		private function __construct(){}
		
		public static function display(){
			if(self::$ob == null)
				return self::$ob = new A();
			else
				return self::$ob;
		}
	}
	
	$a = A::display();
	$b = A::display();
	
	var_dump($a);
	var_dump($b);