PHPOffice / PhpSpreadsheet

        $guess = ($guess === null) ? 0.1 : Functions::flattenSingleValue($guess);

        $rate = $guess;

        if (abs($rate) < self::FINANCIAL_PRECISION) {

            $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;

        } else {

            $f = exp($nper * log(1 + $rate));

            $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;

        }

        $y0 = $pv + $pmt * $nper + $fv;

        $y1 = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;

            if (($nper * abs($pmt)) > ($pv - $fv)) {

                $x1 = abs($x1);

            }

            if (abs($rate) < self::FINANCIAL_PRECISION) {

                $y = $pv * (1 + $nper * $rate) + $pmt * (1 + $rate * $type) * $nper + $fv;

            } else {

                $f = exp($nper * log(1 + $rate));

                $y = $pv * $f + $pmt * (1 / $rate + $type) * ($f - 1) + $fv;

            }

            $y0 = $y1;

            $y1 = $y;