php - Multiplication data from another table in SQL

one text

I have two tables on SQL database, first table is barang, and second table is barang_masuk.

barang :

• id_barang
• nama_barang
• jenis_id
• satuan_id
• harga_barang
• stok

barang_masuk :

• id_barang_masuk
• tgl_masuk
• barang_id
• supplier_id
• jumlah_masuk
• total_harga

I want to inserting data to barang_masuk where total_harga result is obtained from harga_barang * jumlah_masuk.

I'm confused to get the value of total_harga.

this is my code :

function addBarangMasuk()
    {
        $barang_id = $this->input->post('barang_id');
        $jumlah_masuk = $this->input->post('jumlah_masuk');

        $total_harga = $this->db->query("SELECT id_barang, nama_barang, jenis_id, satuan_id, stok, harga_barang * $jumlah_masuk FROM barang WHERE id_barang='$barang_id'");

        $data = [
            'id_barang_masuk' => $this->kode_barang_masuk(),
            'tgl_masuk' => $this->input->post('tgl_masuk'),
            'barang_id' => $barang_id,
            'supplier_id' => $this->input->post('supplier_id'),
            'jumlah_masuk' => $jumlah_masuk,
            'total_harga' =>  $total_harga
        ];

        $this->db->insert('barang_masuk', $data);

        $data_barang = $this->db->query("SELECT * FROM barang WHERE id_barang='$barang_id'")->result();
        foreach ($data_barang as $db) {
            $stok = $db->stok;
            $sisa_stok = $stok + $jumlah_masuk;
        }

        $this->db->query("UPDATE barang SET stok='$sisa_stok' WHERE id_barang='$barang_id'");
    }

Please help me, I am new to MySql