<?php

class RPSN
{
    public $A, $B, $n;

    public function setLoan($A)
    {
        $this->A = $A;
    }

    public function setRepayment($B)
    {
        $this->B = $B;
    }

    public function setNumRepayment($n)
    {
        $this->n = $n;
    }

    public function getValue($in)
    {
        $A = $this->A;
        $B = $this->B;
        $n = $this->n;

        return $A - ($B/pow(1+$in, 1/12)*((pow(1+$in, -1*$n/12)-1)/(pow(1+$in, -1/12)-1)));
    }

    public function calculate()
    {
        $l = 0.000001;
        $r = 2;
        $cOld = -1;
        for ($i = 0; $i <= 50; $i++)
        {
            $c = ($l + $r)/2;
            $lv = $this->getValue($l);
            $rv = $this->getValue($r);
            $cv = $this->getValue($c);

            if ($lv < 0 && $cv < 0 && $rv > 0)
            {
                $l = $c;
            }
            elseif ($lv < 0 && $cv > 0 && $rv > 0)
            {
                $r = $c;
            }
            else
            {
               throw new Exception("Invalid interval");
            }

            if (round($c, 7) == round($cOld, 7))
            {
                return round($c, 7);
            }
            else
            {
                $cOld = $c;
            }
        }
        throw new Exception("Can't solve");
    }
}

$bs = new RPSN;
$bs->setLoan(50000); // půjčka
$bs->setRepayment(949); // měsíční splátka
$bs->setNumRepayment(84); // počet měsíčních splátek
echo $bs->calculate()*100, " %";