<?php

$creditSum = 40000;
$comission = 1000;
$percent = 0.03;
$payment = 5000;
$paymentTotal = 0;

while ($creditSum >= 0)
{
	if ($payment > $creditSum + $comission + $creditSum * $percent)
		$payment = $creditSum + $comission + $creditSum * $percent;
	$creditSum -= $payment - $comission - $creditSum * $percent;
	$paymentTotal += $payment;
}
echo "Всего выплачено {$paymentTotal}";
?>