How can I replicate the Excel FV function in PHP?

Question

I'm converting some rather complex calculations from an Excel spreadsheet, to PHP. I'm stuck on the conversion of Excel's FV function, which is defined thusly:

FV( interest_rate, number_payments, payment, PV, Type )

I've been working on this for 2 hours now, and there has to be something I'm missing. Essentially, I need to replicate this functionality into an equivalent PHP function, taking all of the aforementioned arguments.

Any help would be greatly appreciated.

Answer 1

Slightly modified from the PHPExcel function library:

/**
 * FV
 *
 * Returns the Future Value of a cash flow with constant payments and interest rate (annuities).
 *
 * @param   float   $rate   Interest rate per period
 * @param   int     $nper   Number of periods
 * @param   float   $pmt    Periodic payment (annuity)
 * @param   float   $pv     Present Value
 * @param   int     $type   Payment type: 0 = at the end of each period, 1 = at the beginning of each period
 * @return  float
 */
function FV($rate = 0, $nper = 0, $pmt = 0, $pv = 0, $type = 0) {

    // Validate parameters
    if ($type != 0 && $type != 1) {
        return False;
    }

    // Calculate
    if ($rate != 0.0) {
        return -$pv * pow(1 + $rate, $nper) - $pmt * (1 + $rate * $type) * (pow(1 + $rate, $nper) - 1) / $rate;
    } else {
        return -$pv - $pmt * $nper;
    }
}   //  function FV()


echo FV(0.0149562574418, 4, 43.875, -250);

returns 85.818510876629

// Unit test

class ExcelTest extends \PHPUnit_Framework_TestCase
{

    public function test_it_calculates_fv_value()
    {
        $test_data = [
            [ 0.005,          10, -200,  -500,    1,    2581.4033740601 ],
            [ 0.01,           12, -1000, null,    null, 12682.503013197 ],
            [ 0.009166666667, 35, -2000, null,    1,    82846.246372418 ],
            [ 0.005,          12, -100,  -1000,   1,    2301.4018303409 ],
            [ 0.004166666667, 60, -1000, null,    null, 68006.082841536 ],
            [ 0.025,          16, -2000, 0,       1,    39729.460894166 ],
            [ 0.0,            12, -100,  -100,    null, 1300            ]
        ];

        $test_case_id = 0;
        foreach($test_data as $test_case) {
            $test_case_id++;
            list($rate, $nper, $pmt, $pv, $type, $expected_result) = $test_case;
            $this->assertEquals($expected_result, Excel::FV($rate, $nper, $pmt, $pv, $type), "Test case $test_case_id failed", 0.0000001);
        }
    }

}