PHP read the session of CI3.02

CI:
var_export($_SESSION);

display

array ( '__ci_last_regenerate' => 1446806450, 'group' => 'admin', 'group_user' => 'admin@admin.com', )

===
PHP:
<?php
session_start();
var_export($_SESSION);
?>
display

array ( )



I want to know how to read the session of CI3.02 by PHP.