<?php
function HomoCredit($credit, $pay, $commission, $percent)
{
    $tPay = 0;
    for ($month = 1; $credit > 0; $month++) {
        $debt = round($credit * $percent + $commission);
        if ($debt < $pay) {
            $pay = $debt;
        }
        $credit = $debt - $pay;
        $tPay += $pay;
    }
    return $tPay;
}

function SoftBank($credit, $pay, $commission, $percent)
{
    $tPay = 0;
    for ($month = 1; $credit > 0; $month++) {
        $debt = round($credit * $percent + $commission);
        if ($debt < $pay) {
            $pay = $debt;
        }
        $credit = $debt - $pay;
        $tPay += $pay;
    }
    return $tPay;
}

function StrawberryBank ($credit, $pay, $commission, $percent)
{
    $tPay = 0;
    $open = 7777;
    $credit = $credit + $open;
    for ($month = 1; $credit > 0; $month++) {
        $debt = round($credit * $percent + $commission);
        if ($debt < $pay) {
            $pay = $debt;
        }
        $credit = $debt - $pay;
        $tPay += $pay;
    }
    return $tPay;
}

$HomoCredit = HomoCredit(39999, 5000, 500, 1.04);
$SoftBank = SoftBank(39999, 5000, 1000, 1.03);
$StrawberryBank = StrawberryBank(39999, 5000, 0, 1.02);
echo "\nHomoCredit: $HomoCredit руб.";
echo "\nSoftBank: $SoftBank руб.";
echo "\nStrawberryBank: $StrawberryBank руб.";