PHP: object type lost when retrieve from $_SESSION

I store and object in the $_SESSION array and try to use it on subsequent page calls. In the snippet that displays the data, when I dump the $_SESSION['object'], I see that its type is __PHP_Incomplete_Class and all the methods are lost.

In this snippet I have tried to add
function my_autoloader() {
include ('/srv/www/websites/dokpe/cms/php/MYSQL_Recordset.php');
}
spl_autoload_register('my_autoloader');
but this does not change anything.

Thanks for any help
François