PHP에서 변수를 파괴하는 2가지 방법은 무엇입니까?

class Human { 
  public $name = '张三'; 
  public $gender = null; 
  public function __destruct() { 
      echo &#39;死了!<br />&#39;; 
  } 
} 
$a = new Human(); 
$b = $c = $d = $a;
unset($a);
$d=null;


echo &#39;<hr />&#39;;
var_dump($a);
echo &#39;<hr />&#39;;
var_dump($b);
echo &#39;<hr />&#39;;
var_dump($c);
echo &#39;<hr />&#39;;
var_dump($d);

Notice: Undefined variable: a in /Library/WebServer/Documents/test.php on line 42
NULL
object(Human)#1 (2) { ["name"]=> string(6) "张三" ["gender"]=> NULL }
object(Human)#1 (2) { ["name"]=> string(6) "张三" ["gender"]=> NULL }
NULL 死了!

<?php
$a = 1;
$b = &$a;
unset($a);
var_dump($a); 
var_dump($b);

Notice: Undefined variable: a in E:\amp\apache\htdocs\index.php on line 5
NULL int(1)

<?php        
$a = 1;        
$b = &$a;        
$a=null;        
var_dump($a);        
var_dump($b);

NULL NULL