<?php

 class SimpleClass

 {

     // property declaration

     public $var = 'a default value';

     // method declaration

     public function displayVar() {

         echo $this->var;

     }

 }

 $instance = new SimpleClass();

 $assigned   =  $instance;

 $reference  =& $instance;

 $assigned->var = "aaaaaaa";

 $assigned = null;

 var_dump($instance);

 var_dump($reference);

 var_dump($assigned);

 /*

  输出内容:

  E:\php>php TestClass.php

 class SimpleClass#1 (1) {

   public $var =>

   string(7) "aaaaaaa"

 }

 class SimpleClass#1 (1) {

   public $var =>

   string(7) "aaaaaaa"

 }

 NULL

 总结:

 $assigned, $reference, $assigned三个变量引用的都是同一块内存地址，

 $assigned = null; 之后，断开了指向$assigned的指针,所以$assigned为空，其他的变量不变

  /