adding objects to $SESSION with serialize()

[Daniele Capuano <el.einad@xxxxxxxxx>]

Hi,
I'm developing a web application using moodle, and I'm trying to create a
PHP object tree to be used in $SESSION. Objects are defined as

class foo {
 private $module_name;
 private $sub_modules = array();
}

I have a main module (object) and I use the following function to add
serialized sub modules to such object:

public function add_sub_module(&$mod) {
   $name = $mod->get_mod_name();
   $this->sub_modules[$name] = serialize(&$mod);
}

where the get_mod_name() function simply returns the $module_name private
field. After adding a sub module to the main module, I always update the
main module in $SESSION using serialize($main_module).
Once returned to the page, i restore the main module with
unserialize($SESSION->$main_module_name), and then I call the following
function to retrieve a sub module:

public function &search_sub_module($name='') {;
       foreach($this->sub_modules as $mod_name => $mod) {
          if ($name == $mod_name) {
             return unserialize($mod);
          }

          $obj_mod_file = $mod_name.".php";
          require_once($obj_mod_file);

          $obj_mod = unserialize($mod);
          $modr =& $obj_mod->search_sub_module($name);
          if ($modr != NULL) {
              return $modr;
          }
       }
       return NULL;
}

I found that sub_modules added to the main_module->sub_modules list are
correctly retrieved, but if I add a sub module to a main_module's sub
module, it cannot be get after the main module has been serialized. What do
I mistake?
Please help.

Thanks

Daniele