<?php

error_reporting(-1);

$creditBalance = 39999; 
$monthlyPayment = 5000;
$totalPayment = 0;
$percent = 1.03;
$payment = 1000;

do {
  for($month =1; $month <=20; $month ++) {
  $creditBalance = ( $creditBalance * $percent ) + $payment - $monthlyPayment;
  $paymentTotal = $paymentTotal + $monthlyPayment;
  echo "Месяц {$month} долг {$creditBalance} выплачено {$paymentTotal}\n";
  if (( $creditBalance * $percent ) + $payment <= $monthlyPayment) {
  $monthlyPayment = $creditBalance;
  }
} while ($creditBalance == 0);