What makes __destruct called twice in such a simple PHP code?

Question

<?php

class A
{
    static private $_instance = null;

    static public function Init()
    {   
        self::$_instance = new A();
    }   

    function __construct()
    {   
        echo "__construct\n";
    }   

    function __destruct()
    {   
        var_dump(debug_backtrace());
        echo "__destruct\n";
    }   
}
$a = A::Init();

Normally, we should get following output: (Yes. I got this result in 2 different servers with PHP 5.2.10-2ubuntu6.10 and PHP 5.3.1)

__construct
array(1) {
  [0]=>
  array(5) {
    ["function"]=>
    string(10) "__destruct"
    ["class"]=>
    string(1) "A"
    ["object"]=>
    object(A)#1 (0) {
    }
    ["type"]=>
    string(2) "->"
    ["args"]=>
    array(0) {
    }
  }
}
__destruct

But, on another server with CentOS release 5.7 and PHP 5.2.17, I got this :

    __construct
    array(2) {
      [0]=>
      array(7) {
        ["file"]=>
        string(10) "/tmp/1.php"
        ["line"]=>
        int(7)
        ["function"]=>
        string(10) "__destruct"
        ["class"]=>
        string(1) "A"
        ["object"]=>
        object(A)#1 (0) {
        }
        ["type"]=>
        string(2) "->"
        ["args"]=>
        array(0) {
        }
      }
      [1]=>
      array(6) {
        ["file"]=>
        string(10) "/tmp/1.php"
        ["line"]=>
        int(21)
        ["function"]=>
        string(4) "Init"
        ["class"]=>
        string(1) "A"
        ["type"]=>
        string(2) "::"
        ["args"]=>
        array(0) {
        }
      }
    __destruct
    array(1) {
      [0]=>
      array(5) {
        ["function"]=>
        string(10) "__destruct"
        ["class"]=>
        string(1) "A"
        ["object"]=>
        object(A)#2 (0) {
        }
        ["type"]=>
        string(2) "->"
        ["args"]=>
        array(0) {
        }
      }
    }
    __destruct

Why does the function __destruct called twice here? Especially the first time.

I think there might be something special in the configuration, any suggestion?

Thanks.

==================

PS: This problem is not caused by "Singleton design pattern". Same issue appeared with following code :

<?php
class A
{
    function __construct()
    {   
        echo "__construct\n";
    }

    function __destruct()
    {
        var_dump(debug_backtrace());
        echo "__destruct\n";
    }
}
$a = new A();

@vascowhite: I feel this is not an exact duplicate. Someone who only knows PHP may not understand that example in C++. While the answers may be somewhat similar, the targeted audiences are vastly different. — animuson
– animuson ♦, Commented Dec 10, 2011 at 7:30
FYI, PHP 5.2.17 on mac this code only fires __destruct once. — Tim G
– Tim G, Commented Dec 10, 2011 at 8:35
PHP 5.3.5 on mac also works correctly. Are you sure you have the same code on all servers? — Tim G
– Tim G, Commented Dec 10, 2011 at 8:38
@TimG Thanks for you test and reply. If you turn on the setting zend.ze1_compatibility_mode = On in your php.ini, you can find twice __destruct in the results. This might be a bug, see my answer blow please. — Alix
– Alix, Commented Dec 11, 2011 at 3:06

Alix · Accepted Answer · 2011-12-11 01:57:09Z

4

Finally I find the reason.

It might be a bug in PHP 5.2.x :

If

zend.ze1_compatibility_mode = On

then you can see "__destruct" twice when execute the code I provide in my question.

This issue has been reported in other versions : https://bugs.php.net/bug.php?id=29756

Not affect PHP 5.3 in my test. (PHP 5.3 removed this setting)

Hope this answer will be helpful for some guys later :)

answered Dec 11, 2011 at 1:57

Alix

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