Regular Payment at Regular Interval (PMT)

pmt.php

<?php

/**
 * Future Value Interest Factor (FVIF)
 *
 * @param float $interest annual interest
 * @param int $periods periods
 * @throws InvalidArgumentException if $interest is not float
 * @throws InvalidArgumentException if $periods is not int
 * @return float future value interest factor
 */
function fvif($interest, $periods)
{
	if(!is_float($interest))
	{
		throw new InvalidArgumentException('$interest is not float');
	}

	if(!is_int($periods))
	{
		throw new InvalidArgumentException('$periods is not int');
	}

	if($periods == 0)
	{
		return 1;
	}

	$interest_factor = $interest + 1;

	if($interest_factor == 0)
	{
		return 0;
	}

	if($periods == 1)
	{
		return $interest_factor;
	}

	if($periods < 0)
	{
		$periods = -$periods;

		$interest_factor = 1 / $interest_factor;
	}

	return pow($interest_factor, $periods);
}

/**
 * Regular Payment at Regular Interval (PMT)
 *
 * @param float $interest annual interest
 * @param int $periods periods
 * @param float $present_value present value
 * @param float $future_value future value
 * @throws InvalidArgumentException if $interest is not float
 * @throws InvalidArgumentException if $periods is not int
 * @return float regular payment at regular interval
 */
function pmt($interest, $periods, $present_value, $future_value)
{
	if(!is_float($interest))
	{
		throw new InvalidArgumentException('$interest is not float');
	}

	if(!is_int($periods))
	{
		throw new InvalidArgumentException('$periods is not int');
	}

	if($interest == 0)
	{
		$pmt = -1 * ($future_value + $present_value) / $periods;
	}
	else
	{
		$n = ($present_value + $future_value);

		$d = fvif($interest, $periods) - 1;

		$pmt = -1.0 * (($n / $d) + $present_value) * $interest;
	}

	return $pmt;
}

?>