you may can do following

$user = ossn_user_by_guid($vars['user']->guid);

Still working on my identity manager, i managed the registration part with the hook you provided.

I am now working on the login hook in OssnUser.php, It seems the user's password is not passed along with the object to the hook :

In the public function Login() i can read :

unset($user->password);
unset($user->salt);
OssnSession::assign('OSSN_USER', $user);
$this->update_last_login();
$vars         = array();
$vars['user'] = $user;
$login        = ossn_call_hook('user', 'login', $vars, true);
return $login; 

And from inside a hook function i wrote, when i print_r the $vars array i get :

[HOOK]user
[TYPE]login
[RETURNVALUE]1
[PARAMS]Array
(
    [user] => OssnUser Object
        (
            [guid] => 1
            [type] => admin
            [username] => BobMoran
            [email] => [email protected]
            [first_name] => Bob
            [last_name] => Moran
            [last_login] => 1571037608
            [last_activity] => 1571037684
            [activation] =>
            [time_created] => 1571037602
            [fullname] => Adrien Allemand
            [birthdate] => 08/08/1908
            [gender] => male
            [data] => stdClass Object
                (
                )
        )
)

I need access to that password to plugin my identification manager. Is there a nicer way than selecting it from DB in my hook ?

Yes you may stop user creation.

ossn_add_hook('user', 'create', 'disable_user_create');
function disable_user_create(){
       return false;
}

Is there a way to prevent user creation/modification from the callbacks ? I need to prevent user creation if my microservice doesn't respond or raises an exception. I am thinking of a hook of some sort able to stop the user creation or update process ossn-side ?

ok thanks !

For registration you may use

ossn_register_callback('user', 'created', 'my_callback_abc');

For others you may use callback that arises before the action is performed,

ossn_register_callback('action', 'load', 'my_action_callback');
function my_action_callback($callback, $type, $vars){
         if($vars['action'] == 'resetlogin'){
                 //
        }
}