$_SESSION keys for Solar classes match the class name. Thus, a Acme_Example class using a session variable called foo would be addressed as $_SESSION['Acme_Example']['foo'].

The easiest way to adhere to this convention is to use a Solar_Session storage object within your class. For example ...

<?php
$session = Solar::factory('Solar_Session', array(
    'class' => 'Acme_Example',
));

// equivalent of $_SESSION['Acme_Example']['something']
$session->store['foo'] = 'bar';