You normally use sessions in PHP to store information about the user’s current session that will help you track him while he is browsing your website.
Sometimes you will want to destroy that session or all the sessions registered to that user, for example when he wants to log out.

The easiest way of destroying all sessions for that user is to use the session_destroy() function:

<?php
session_destroy();
?>

You should note that if you are planning to use this function after the headers have been sent to the user, the session variables will still exist until a new page will load (or new headers are sent to the client). So, after you use session_destroy() you will need to redirect the user to a new page so that the new headers are sent or simply make sure that you use the function before you send any headers to the users (headers are sent automatically when you send output through echo, print() or even plain text/html in the file).

Furthermore, you should be calling session_start() before using anything that involves sessions, to make sure the session handling is started on the server.