Interest Rate Calculation Script for PHP / Javascript

Here's an interest rate calculation script using Newton's Method, I couldn't find a similar function in PHP or Javascript.

PHP (View demo)

# Interest Rate Calculation
function interestrate($years, $payment, $loan_amount, $paymentperyear)
{
$l_result = "";
# Interest rate guess
$interest = .1;

$g[0] = $interest / $paymentperyear;

for ($i=0;$i<10;$i++) {
$numerator = $payment - $payment * pow(1 + $g[$i], -$years) - $g[$i] * $loan_amount;
$denominator = $years * $payment * pow(1 + $g[$i], -$years - 1) - $loan_amount;
$g[$i + 1] = $g[$i] - $numerator / $denominator;
}

return $g[$i];
}

Javascript (View demo)

// Interest Rate Calculation
function interestrate(years, payment, loan_amount, paymentperyear)
{
var l_result = "";
var numerator, denominator, g = new Array;
// Interest rate guess var interest = 0.1;
g[0] = interest / paymentperyear;
for (i=0;i<10;i++)
{
numerator = payment - payment * Math.pow(1 + g[i], -years) - g[i] * loan_amount;
denominator = years * payment * Math.pow(1 + g[i], -years - 1) - loan_amount; g[i + 1] = g[i] - numerator / denominator; }
return g[i];
}

function doCalc()
{
var f = document.FrmCalc;
f.interest.value = interestrate(f.years.value, f.payment.value, f.loan_amount.value, f.paymentperyear.value);
}