Codeignitter form validation with Callback function

FormValidWithCallback.php

public function check_unique() {
		$conf_group = $this->input->post('conf_group');
		$label = $this->input->post('label');
		$code =  strtolower(str_replace(' ', '_', $label));
		
		$this->db->select('code');
		$this->db->from('acc_meta_conf');
		$this->db->where('code', $code);
		$this->db->where('conf_group', $conf_group);
		$query = $this->db->get();
		$num = $query->num_rows();
		if ($num > 0) {
			$this->form_validation->set_message('check_unique', 'Name is already taken');
			return FALSE;
		} else {
			return TRUE;
		}
	}


	function savePublicationConfig($conf_id = NULL){

		$this->form_validation->set_rules('label', 'Name', 'callback_check_unique');

		if ($this->form_validation->run() == FALSE) {
			echo json_encode(array('error' => validation_errors()), JSON_HEX_TAG);
		}else{
			$this->PublicationConfig_mdl->savePublicationConfig($conf_id);
		}

	}