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

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 :)