Excel PMT in PHP and JavaScript

pmt.js

/**
 * Copy of Excel's PMT function.
 * Credit: http://stackoverflow.com/questions/2094967/excel-pmt-function-in-js
 *
 * @param rate_per_period       The interest rate for the loan.
 * @param number_of_payments    The total number of payments for the loan in months.
 * @param present_value         The present value, or the total amount that a series of future payments is worth now;
 *                              Also known as the principal.
 * @param future_value          The future value, or a cash balance you want to attain after the last payment is made.
 *                              If fv is omitted, it is assumed to be 0 (zero), that is, the future value of a loan is 0.
 * @param type                  Optional, defaults to 0. The number 0 (zero) or 1 and indicates when payments are due.
 *                              0 = At the end of period
 *                              1 = At the beginning of the period
 * @returns {number}
 */
function pmt(rate_per_period, number_of_payments, present_value, future_value, type){
    future_value = typeof future_value !== 'undefined' ? future_value : 0;
    type = typeof type !== 'undefined' ? type : 0;

	if(rate_per_period != 0.0){
		// Interest rate exists
		var q = Math.pow(1 + rate_per_period, number_of_payments);
		return -(rate_per_period * (future_value + (q * present_value))) / ((-1 + q) * (1 + rate_per_period * (type)));

	} else if(number_of_payments != 0.0){
		// No interest rate, but number of payments exists
		return -(future_value + present_value) / number_of_payments;
	}

	return 0;
}

pmt.php

/**
 * Copy of Excel's PMT function.
 * Credit: http://thoughts-of-laszlo.blogspot.nl/2012/08/complete-formula-behind-excels-pmt.html
 *
 * @param double $interest        The interest rate for the loan.
 * @param int    $num_of_payments The total number of payments for the loan in months.
 * @param double $PV              The present value, or the total amount that a series of future payments is worth now;
 *                                Also known as the principal.
 * @param double $FV              The future value, or a cash balance you want to attain after the last payment is made.
 *                                If fv is omitted, it is assumed to be 0 (zero), that is, the future value of a loan is 0.
 * @param int    $Type            Optional, defaults to 0. The number 0 (zero) or 1 and indicates when payments are due.
 *                                0 = At the end of period
 *                                1 = At the beginning of the period
 *
 * @return float
 */
function PMT($interest,$num_of_payments,$PV,$FV = 0.00, $Type = 0){
	$xp=pow((1+$interest),$num_of_payments);
	return
		($PV* $interest*$xp/($xp-1)+$interest/($xp-1)*$FV)*
		($Type==0 ? 1 : 1/($interest+1));
}

@maarten00: Great one! Thanks for your work.
Do you know if there is an issue with the two functions and the round-options?
I do get two slightly different results in several use cases, when comparing the function's result.

Do you have any idea where that issue comes from? Probably a matter of decimal places? I'm talking about like 1 up to 3 $ difference... it's not much, but still, it seems to be a different value.

Thanks for your feedback.

I have experienced some very slight differences in the output when comparing it to the output from Excel. In my case the differnce was negligible because I was only displaying prices rounded to 2 decimals. I don't remember if I looked into the issue or not, but I did add the original links where I found the code. Maybe someone there had the same issue and added an updated version :)

http://stackoverflow.com/questions/2094967/excel-pmt-function-in-js
http://thoughts-of-laszlo.blogspot.nl/2012/08/complete-formula-behind-excels-pmt.html

This just saved me a lot of time. Thanks Man

for pmt.js - line 23, you are dividing the number with base using variable type, and type by default is 0 which lead to number/0 and become infinity number. It looks like a different formula between js and php.

Hi!
This function is great, thank you!
I am comparing this to a function in an excel sheet, and it seems like if the fv is not 0, the results differ from excel and this function.
Do you know what causes this?

Example:
Formula Excel
r 0 0
n 10 10
pv 8000 8000
fv 1000 1000
type 0 0

Result -900 700

If fv is 0:
Result -800 800