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