Graph generating compound interest

<?php

$graphingFunction = function($months, $paymentPerMonth) {

    $addPaymentAndInterest = function ($payment) use ($paymentPerMonth) {
        static $last = 0;
        $interestRate = (rand(0, 100) / 20) / 100;
        $last =  $paymentPerMonth + ($last * (1 + $interestRate));
        return $last;
    };

    return array_map($addPaymentAndInterest, array_fill(0, $months, $paymentPerMonth));
};

$graph = $graphingFunction(50, 100);

var_dump($graph);