如何使用php mysqli将数据插入数据库之前进行计算

I want to calculate the bond balance using amount saved in the database and bond amount inputted on a form then insert the result to the database plus the other information. am using php and mysql. below is the code I have

if (isset($_POST['submit'])) {
    $ref_no = mysqli_real_escape_string($dbc, $_POST['ref_no']);
    $cargo_des = mysqli_real_escape_string($dbc, $_POST['cargo_des']);
    $te_no = mysqli_real_escape_string($dbc, $_POST['te_no']);
    $bond = mysqli_real_escape_string($dbc, $_POST['bond']);
    $bond_no = mysqli_real_escape_string($dbc, $_POST['bond_no']);
    $bond_amount = mysqli_real_escape_string($dbc, $_POST['bond_amount']);
    $date_lodged = mysqli_real_escape_string($dbc, $_POST['date_lodged']);
    //$bond_balance = mysqli_real_escape_string($dbc, $_POST['bond_balance']);
    $query = mysqli_query ($dbc, "SELECT amount FROM bond");
    while( $row = mysqli_fetch_array($query) ) {
        echo $row['amount'];
    }
    $bond_balance = 0;
    $examout = $row['amount'];
    $amount = $_POST['bond_amount'];
    $bond_balance = $examout - $amount;
    $q = "INSERT INTO new_bond (ref_no, cargo_des, te_no, bond, bond_no, bond_amount, date_lodged) "
            . "VALUES ('$ref_no','$cargo_des','$te_no','$bond','$bond_no','$bond_amount','$date_lodged','$bond_balance')";
    $r = mysqli_query($dbc, $q) or die(mysqli_error($dbc));
    if ($r) {
        echo '<script>alert("Success")</script>';
        echo "<script>window.open('?page=new_bond','_self')</script>";
    }
    exit();
}