php 静态变量

 1<?php
 2  class User
 3  {
 4           private static $counter = 0;
 5          
 6           public function __construct()
 7           {
 8                    self::$counter++;
 9           }
10           public function getCount()
11           {
12                   return self::$counter;
13           }
14          
15           public function __destruct()
16           {
17                    self::$counter--;
18           }
19  }
20  $user1 = new User();
21  $user2 = new User();
22  $user3 = new User();
23  echo $user1->getCount().'<br>';
24  unset($user1);
25  echo $user2->getCount();
26  
27?>