<?php
error_reporting(-1);
$creditSum = 40000;
$comission = 1000;
$payout    = 5000;
$payment   = 0;
for ($month = 1; $month <= 15; $month++) {
    $creditSum = $creditSum * 0.03 + $creditSum + $comission - $payout;
    $payment   = $month * $payout;
    echo "{$month} месяц спустя: долг равен  {$creditSum}, оплачено {$payment}.\n";
    if ($creditSum * 0.03 + $creditSum + $comission < 5000) {
        $month++;
        $payment   = $payment + $creditSum;
        $creditSum = $creditSum - $creditSum;
        echo "{$month} месяц спустя: долг равен  {$creditSum}, оплачено {$payment}.\n";
        
        break;
    }
}