What does session_destroy() do in PHP?

Question

In PHP manual, the description for session_destroy() function is :

session_destroy() destroys all of the data associated with the current session. It does not unset any of the global variables associated with the session, or unset the session cookie. To use the session variables again, session_start() has to be called.

I am confused about this description. If this function destroys all session data, then why the global variables associated with the session are not unset? Why can we use the session variables again?

I think you missed the following para of the same page : In order to kill the session altogether, like to log the user out, the session id must also be unset. If a cookie is used to propagate the session id (default behavior), then the session cookie must be deleted. setcookie() may be used for that. — Satya
– Satya, Commented Aug 31, 2013 at 14:29
xIf this function destroys all session data, then why the global variables associated with the session are not unset? Because there is no point in doing so. The global variables will be gone upon the next request. — Pekka
– Pekka, Commented Aug 31, 2013 at 14:29
The global variables you think that are associated with that session are not associated with that session any longer because the session has already been destroyed. You can use the variables as variables but not as session variables. There is no reason to destroy the variables as well, so you probably should add to your question why you think otherwise so it's more clear what you ask about. Perhaps there is some concrete problem you need help with? — hakre
– hakre, Commented Aug 31, 2013 at 15:04

hakre · Accepted Answer · 2013-08-31 14:59:07Z

I am confused about this description. If this [session_destroy()] function destroys all session data, then why the global variables associated with the session are not unset? Why can we use the session variables again?

Session data is the data associated with the session. The session is defined by its name (the session name) and its id (the session id).

By using that function, all this sessions (name + id) data is destroyed.

The variable container which allowed you to read / set that data is still there, so you can operate on that data (e.g. there might be information in like last activity and this is a logout and you want to store the last activity know at logout or so into some logs or database, so why delete it? that would be counter productive because you want to destroy (or commit) sessions fast, e.g. when you know read-only access is needed only, keep the session data in memory, but commit the session already because there is no need to keep it open).

Keep in mind that even these variables are access via $_SESSION they are not part of the session any longer. Perhaps that is the confusing part?

BTW my description is not totally correct. PHP internally identifies the session data by the id only so you could change the session name and session_destroy() would still remove the session data because the session id has not changed.

Thank you for your answer. I misunderstood the description initially. I messed up the session data with session variables.

score 2 · Accepted Answer · 2013-08-31 15:20:41Z

session_destroy() deletes the session file where session data are stored. Look here:

<?php session_save_path('./session/'); session_start(); $_SESSION['v'] = array( 'foo' => 123, 'bar' => 'spam' ); $_SESSION['m'] = "rocky"; if( isset($_GET['delete']) == 'true' ) session_destroy(); ?>

I have a script whitch creates a session and set the value of v to 10, and it saves the session data in the same script path in a folder named ./session.

Now open the page and then browse the ./session directory, you should see a file with name similar to sess_4r7ldo7s5hsctu3fgtvfmf4sd0. This is where session data is being stored and it will contains:

v|a:2:{s:3:"foo";i:123;s:3:"bar";s:4:"spam";}m|s:5:"rocky";

Activate session_destroy() by passing ?delete=true to the page, the session file will be simply deleted.

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