如何使用CodeIgniter更新MySQL中的数学设置？

Question

I want to update data stock by adding the current data with the inputted data. What is the correct way?

public function addStock($tabel,$total,$id_barang){
    $this->db->set("total","total + $total");
    $this->db->where('id_barang',$id_barang);
    $this->db->update($tabel);  
}

The output should be the result of the addition, but it keeps returning 0.