I can't change PHP session id

I am trying to change session id by using

session_unset();

and

session_destroy();

after I use session_start().

But I print session_id() after each session destroy and it didn't change in all browsers: IE8, Chrome and Firefox.

How is this caused and how can I solve it?

Try using the session_regenerate_id function: http://php.net/manual/en/function.session-regenerate-id.php

Try this from the PHP documentation :

session_start();
$_SESSION['destroyed'] = time();
session_regenerate_id();
unset($_SESSION['destroyed']);

http://php.net/manual/en/function.session-regenerate-id.php