<?php

error_reporting(-1);

function getSum ($percent, $tax, $account, $creditSum, $payout) {
	$paymentTotal = 0;
	$creditSum = $creditSum + $account;
	for ($i ; $i < 100 ;$i++) {
		$creditBalance = $creditSum * $percent - $payout + $tax;
		//надо подумать над другим ограничением цикла
		$paymentTotal +=  $payout;
		$creditSum = $creditBalance;
		if ($creditBalance < 0) {
			$paymentTotal += $creditBalance;
			break;
		}
	}
	return $paymentTotal;
}
$creditSum = 39999;
$payout = 5000;

$homoCreditBank = getSum (1.04, 500, 0, $creditSum, $payout);
$softBank = getSum (1.03, 1000, 0, $creditSum, $payout);
$StrawberryBankTotal = getSum (1.02, 0, 7777, $creditSum, $payout);

echo "homoCreditBank {$homoCreditBank} руб \n";
echo "SoftBank {$softBank} руб \n";
echo "StrawberryBankTotal {$StrawberryBankTotal} руб \n";