Problem iwith validation callback

Solved,

In fact this is not possible :
$this->form_validation->set_rules('annee', 'Année', 'required|callback_annee_check');

You must write this
$this->form_validation->set_rules('annee', 'Année', 'callback_annee_check');
And create the error message manually in the callback function in case of empty field.