Form validation callback return value

[eluser]Anonymous[/eluser]
hi there,

Anyone knows why my callback return value is being ignored ?

Code:

$this->form_validation->set_rules('groningen', 'groningen', 'callback_username_check');

the callback

Code:

function username_check() {    
        return 'testing';
    }

Code:

var_dump($this->input->post('groningen'));

gives my: boolean false, instead of: string 'testing' as what it is supposed to do.