<?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