php中单例模式的解析说明

<?php   
      //单例模式
class Dbconn{
        private static $_instance=null;
        protected static $_counter=0;
        protected $_db;
        private function __construct(){
            self::$_counter+=1;
        }
        public  static function getInstance(){
            if(!self::$_instance instanceof self){
                self::$_instance=new self();
            }
            return self::$_instance;
        }
        public function connect(){
            echo "connected:".(self::$_counter)."\n";
            return $this->_db;
        }    
}
////使用单例模式后不能直接new对象，必须调用getInstance获取
$conn1=Dbconn::getInstance();
$_db=$conn1->connect();
//第二次调用是同一个实例，_counter还是1
$conn2=Dbconn::getInstance();
$_db=$conn2->connect();
?>
结果显示：connected:1 connected:1