<?php

error_reporting(-1);

$creditBalance  = 40000;
$percent        = 1.03;
$servicePayment = 1000;
$monthlyPayment = 5000;
$paymentTotal   = 0;


for ($month = 1; $month <= 20; $month++) {
    
    $creditBalance = ($creditBalance * $percent) + $servicePayment;
    
    if ($creditBalance < $monthlyPayment) {
        $monthlyPayment = $creditBalance;
        $creditBalance  = $creditBalance - $monthlyPayment;
        
    }
    
    if ($creditBalance > $monthlyPayment) {
        $creditBalance = $creditBalance - $monthlyPayment;
    }
    
    $paymentTotal = $paymentTotal + $monthlyPayment;
    echo "{$month} месяц спустя: долг = {$creditBalance} руб, выплачено всего {$paymentTotal} руб. \n";
    
    if ($creditBalance <= 0) {
        break;
        
    }
}