Codeigniter passing 2 arguments to callback – Email validation

$id = 2;
$this->load->library('form_validation');
        if ($this->input->server('REQUEST_METHOD') == 'POST') {
            $this->form_validation->set_rules('user_email', 'Email', 'min_length[5]|valid_email|required|xss_clean|callback_is_unique_email['.$id.']',array('required'=> 'You have not provided %s.','is_unique'=> 'This %s already exists.'));
            $this->form_validation->set_message('is_unique_email', 'Email Address already in use');
            $this->form_validation->set_message('valid_email', 'Please enter valid email address');
if ($this->form_validation->run() == FALSE) {
echo 'error';
} else {
echo 'success';
}
}

function is_unique_email($email, $id) {
        $this->db->where('user_email', $email);
        $this->db->where('id !=', $id);
        $query = $this->db->get('users');
        if($query->num_rows() > 0) {
            return false;
        } else {
            return true;
        }
    }