[imp] Change conf value after login

[Marco]

Hi,
I explain in particular what I would like.
I need an hook that set, after login, the IMP configuration variable
$conf['compose']['link_attachments'].

I tried with:

  if (!function_exists('_imp_hook_postlogin')) {
      function _imp_hook_postlogin($actionID, $isLogin)
      {
         global $registry;
         			
 
$GLOBALS['registry']->_confCache['imp']['compose']['link_attachments'] = 
true;

         return true;
      }
  }

..but it doesn't work (/var/log/httpd/ssl_error_log.29:[Fri Jun 12 14:42:08
2009] [error] [client 19.19.19.19] PHP Fatal error:  Cannot use object 
of type
Registry as array in 
/var/www/html/horde-webmail-1.2.3/imp/config/hooks.php on
line 49).

Could you show me how to set this value?

Thank you very much
Best Regards
marco