Finding NPV (net present value) with PHP

Hello Experts!

I need some help finding NPV from an array of reruns.

Let say i have an array containing 20 years of payments with array[0] being the capital outlay.

Array ( [0] => -29669.45 [1] => 84862.5 [2] => 84913.414741969 [3] => 84964.360031128 [4] => 85015.335885805 [5] => 85066.342324338 [6] => 85117.379365077 [7] => 85168.447026381 [8] => 85219.545326622 [9] => 85270.674284183 [10] => 85321.833917456 [11] => 85373.024244847 [12] => 85424.245284771 [13] => 85475.497055654 [14] => 85526.779575933 [15] => 85578.092864059 [16] => 85629.436938489 [17] => 85680.811817695 [18] => 85732.21752016 [19] => 85783.654064375 [20] => 85835.121468845 [21] => 354928.55 ) 

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Now... let say i wanted to find the net present value in year 10 with a ROR or discount rate of 8.15%

How would I do that?

My current script:
function npv($rate, $values, $year) {
    for ($i=0;$i<=$year;$i+=1) {
        $npv = $values[$year - $i] + $npv / (1 + $rate);
    }
    return '$'.number_format($npv,2,'.',',');
}

echo npv(.0815, $paymentArray, 10);

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Any Ideas would be awesome!
LVL 2
CybervanesAsked:
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TommySzalapskiConnect With a Mentor Commented:
Ah, I see it. The value that's 10 years old needs to be divided by the rate 10 times. Or (1+rate)^10
If you do this
$npv = ($values[$year - $i] + $npv) / (1 + $rate);
then it should work, since you are already running backward through the array.

By the way, I would do the for loop like this, but it either way is the same.
    for ($i=$year;$i>=0;$i-=1) {
        $npv = ($values[$i] + $npv) / (1 + $rate);
    }

Actually, if array[0] is the capital outlay, shouldn't you run through starting at 0 and going to $year?
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CybervanesAuthor Commented:
I'm sorry this should have been worth 500 points...
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CybervanesAuthor Commented:
That was no help. Unless I'm missing somthing... Did you look at that page?
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TommySzalapskiCommented:
In your code is $rate 8.15 or .0815? It needs to be .0815; if it isn't use $rate/100.
Also, you should not be dividing by (1+$rate) you should multiply by (1-$rate).
If I understand the question correctly, that should be all you need to get it working right.
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TommySzalapskiCommented:
Scratch that second part. The division is fine. What is your code doing that you think it shouldn't?
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CybervanesAuthor Commented:
I'm noting getting the same figures that i do when using excel's internal NPV function.
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TommySzalapskiCommented:
Er... That last line might not make sense. I don't know NPV that well, I'm just good at figuring formulas out. Doing it the way I suggested matched Excel perfectly.
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CybervanesAuthor Commented:
Thanks!
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