Sessions are stored server-side, so you cannot add values ​​to them from JavaScript. On the client side you can only get the session cookie, which contains an id. One possibility is to send an AJAX request to a server-side script, which will set the session variables. Here is an example using jQuery's .post() method:

$.post('/setsessionvariable.php', { name: 'value' });

Of course, you should be careful about exposing such scripts.

In JavaScript:

jQuery('#div_session_write').load('session_write.php?session_name=new_value');

In the session_write.php file:

<?
session_start();

if (isset($_GET['session_name'])) {$_SESSION['session_name'] = $_GET['session_name'];}
?>

In HTML:

<div id='div_session_write'> </div>