PHP單態模式簡單用法

<?php
/*
作者 : shyhero
*/
class Demo{
    private static $ob=null;
    private $name;
    private $age;
    private function __construct(){
    }
    public static function makeOb(){
      var_dump($a);
      if(self::$ob == null){
        $c = __CLASS__;
        self::$ob = new $c();
      }
      return self::$ob;
    }
    public function __set($tName,$value){
      $this -> $tName = $value;
    }
}
$a = Demo::makeOb();
var_dump($a);