PHP Mimic Excel Rate Function

Teddy Zugana

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Oct 14, 2022

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<?php
define('FINANCIAL_MAX_ITERATIONS', 128); 
define('FINANCIAL_PRECISION', 1.0e-08); 

function RATE($nper, $pmt, $pv, $fv = 0.0, $type = 0, $guess = 0.1) 
{

        $rate = $guess;
        if (abs($rate) < 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;

        // find root by secant method
        $i  = $x0 = 0.0;
        $x1 = $rate;
        while ((abs($y0 - $y1) > FINANCIAL_PRECISION) && ($i < FINANCIAL_MAX_ITERATIONS)) {
            $rate = ($y1 * $x0 - $y0 * $x1) / ($y1 - $y0);
            $x0 = $x1;
            $x1 = $rate;
            if (($nper * abs($pmt)) > ($pv - $fv))
                $x1 = abs($x1);

            if (abs($rate) < 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 = $y;
            ++$i;
        }
        return $rate;
    }   //  function RATE()

$nper = 240;
$pmt = 3250000;
$pv = -300000000;

$test = RATE($nper, $pmt, $pv);

echo $test*100;
?>