今天的学习内容：
魔术方法创建单个PDO对象
单例实例
namespace via\web;
class Cookie
{
private static $_instance=null;
private function __construct(){
}
public static function singleton(){
if(self::$_instance==null){
self::$_instance=new self();
}
if(!(self::$_instance instanceof self)){
self::$_instance=new self();
}
return self::$_instance;
}
public function __clone(){
throw new \Exception('不能克隆这个对象');
}
}
