SugarCRM invoice add logic hook to display price detail - php

Question

When creating an invoice with SugarCRM, I would like to add a line to the invoice header section displaying details of the total amount of the invoice (VAT included and VAT excluded) total amount. The two values can be calculated using the price keyed in in the invoice detail section.
I am told to implement this functionality using the logic hook function of SugarCRM. Since I am new to SugarCRM, could you please advise ?
This is what I have done so far :
<?php
class AddLines {
function update_amount(&$bean, $event, $arguments)
{
//Query c_inv_fee_distribution table for detail_amount value of parent
$bean->client_po = 'working';
}
}
class PriceCalc {
function taxdetail(&$bean, $event, $arguments)
{
$total_amt_excl = 0;
$total_amt_incl = 0;
$line_price = $number_units * $unit_price;
$invoicelength = count($bean->invoice);
for ($line_number=0; $line_number < $invoicelength); $line_number++)
{
if $VAT = true {
$total_amt_excl += $line_price[line_number];
$total_amt_incl += $line_price[line_number] * 1.14;
}else {
$total_amt_excl += $line_price[line_number];
$total_amt_incl += $line_price[line_number];
}
}
}
?>