php中单例模式的解析说明

 1 <?php   
 2       //单例模式
 3 class Dbconn{
 4         private static $_instance=null;
 5         protected static $_counter=0;
 6         protected $_db;
 7         private function __construct(){
 8             self::$_counter+=1;
 9         }
10         public  static function getInstance(){
11             if(!self::$_instance instanceof self){
12                 self::$_instance=new self();
13             }
14             return self::$_instance;
15         }
16         public function connect(){
17             echo "connected:".(self::$_counter)."\n";
18             return $this->_db;
19         }    
20 }
21 ////使用单例模式后不能直接new对象，必须调用getInstance获取
22 $conn1=Dbconn::getInstance();
23 $_db=$conn1->connect();
24 //第二次调用是同一个实例，_counter还是1
25 $conn2=Dbconn::getInstance();
26 $_db=$conn2->connect();
27 ?>
28 结果显示：connected:1 connected:1