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(); 

Answer 1

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 :)