<?php

class Credit
{
    private static $interest_rate;
    private $payment;
    private $current_sum;
    private $months_count = 0;
    private $paid = 0;
    private $percent = 0;

    /**
     * @param double $interest_rate - процентная ставка
     */
    public function __construct($interest_rate)
    {
        self::$interest_rate = $interest_rate;
    }

    /**
     * Производит ежемесячный платеж и начисление процентов на остаток
     * @return void
     */
    private function month()
    {
        $this->months_count += 1;
        $this->percent += $this->current_sum * self::$interest_rate;
        $current_sum_part = $this->current_sum / ($this->current_sum +
            $this->percent);
        if(($this->current_sum + $this->percent) > $this->payment){
            $this->paid += $this->payment;
            $this->current_sum -= $current_sum_part * $this->payment;
            $this->percent -= (1 - $current_sum_part) * $this->payment;
            $this->month();
        } else {
            $this->paid += $this->current_sum + $this->percent;
        }
    }

    /**
     * @param int $credit_amount - общая сумма кредита
     * @return double
     */
    private function res($credit_amount)
    {
        $this->current_sum = $credit_amount;
        $this->month();
        return $this->paid;
    }

    /**
     * @param int    $credit_amount - общая сумма кредита
     * @param int    $payment       - ежемесячный платеж
     * @return void
     */
    public function out($credit_amount, $payment)
    {
        $this->payment = $payment;
        $overpayment = $this->res($credit_amount) - $credit_amount;
        echo 'Переплата составила: ' . $overpayment . '$ <br>';
        echo 'Кредит был погашен через ' . $this->months_count . ' месяцев. <br>';
    }
}

(new Credit(0.07))->out(400, 40);