Loan Calculating late payment with daily 0.2% on arrears - php

Question

I am trying to calculate a late daily fee of 0.02%, but when the second installment is reached the late fee adds the second installment plus total fee from first installment and now calculate daily fee with new accrued figure.
Example
First installment 5 days late before reaching second instalment:
fee=(arrears*5*0.002)
Second instalment is reached and loan overdue for another 5 days:
fee2=((instalment2+fee)*5*0.002))

Answer 1

Usually the interest use Compound Interest the formula (arrears*(1+0.002)^5), not Simple Interest formula (arrears*5*0.002).
Anyway, I made a piece of code to show how can you do it.
<?php
function CalculateLoan($Value,$Days,$Fee){
//Simple Interest
return $Value + $Value*$Days*$Fee;
//Compound interest
//return $Value * pow(1+$Fee,$Days);
}
// Assuming 6 Instalments of $1000 every 5 days.
$ArrayInstalments = array();
$ArrayInstalments[0]= array("Value"=>1000,"PayDay"=>0);
$ArrayInstalments[1]= array("Value"=>1000,"PayDay"=>5);
$ArrayInstalments[2]= array("Value"=>1000,"PayDay"=>10);
$ArrayInstalments[3]= array("Value"=>1000,"PayDay"=>15);
$ArrayInstalments[4]= array("Value"=>1000,"PayDay"=>20);
$ArrayInstalments[5]= array("Value"=>1000,"PayDay"=>25);
// 30 days passed
$CurrentDay = 30;
// 0.2% fee/day
$Fee = 0.002;
// Initial Debit
$Sum = 0;
foreach ($ArrayInstalments as $key=>$Instalment){
$ThisDebit = CalculateLoan($Instalment['Value'],max($CurrentDay-$Instalment['PayDay'],0),$Fee);
echo 'Instalment:'.$key.' Debit:' . $ThisDebit . '<br/>';
$Sum += $ThisDebit;
}
echo 'Total:' .$Sum. '<br/>';
?>