Simple Rolling DRIP Estimator

estimate.php

<?php
/*

This script will take into account your current positions and calculate your dividend reinvestment over time.

@todo Pull positions from somewhere, a local json file maybe?
@todo Pull live-ish position data so stockPrice isn't stale. 
@todo Use of CLI arguments 
@todo There is a bug with addMonthly that really inflates the number of shares
@todo Fractional shares on addMonthly?
@todo Turn into a class... slipper slope

*/
 

$positions = [
	'SRET' => ['stockPrice' => 8.73, 'dividendTerm' => 'monthly', 'dividendPercent' => 8.88, 'shares' => 158.903, 'addMonthly' => 0],
	'SPY' => ['stockPrice' => 369.06, 'dividendTerm' => 'quarterly', 'dividendPercent' => 1.52, 'shares' => 4.032, 'addMonthly' => 0],
];

// Calculate intial values
foreach($positions as &$pos){
	$pos['value'] = number_format($pos['stockPrice'] * $pos['shares'], 2, '.', '');
}
unset($pos);

function monthlyDividends(&$positions) {
	foreach ($positions as &$position) {
		if ($position['dividendTerm'] == 'monthly') {
			$dividendPercent = $position['dividendPercent'] / 100 / 12;
			$dividendPaymentTotal = $position['shares'] * ($position['stockPrice'] * $dividendPercent);
			$position['shares'] += number_format($dividendPaymentTotal / $position['stockPrice'], 3, '.', '');
			$position['value'] = number_format($position['stockPrice'] * $position['shares'], 2, '.', '');
		}
	}
	unset($position);
}


function quarterlyDividends(&$positions) {
	foreach ($positions as $symbol => &$position) {
		if ($position['dividendTerm'] == 'quarterly') {
			$dividendPercent = $position['dividendPercent'] / 100 / 4;
			$dividendPaymentTotal = $position['shares'] * ($position['stockPrice'] * $dividendPercent);
			$newShares = number_format($dividendPaymentTotal / $position['stockPrice'], 3, '.', '');
			$position['shares'] += $newShares;
			$position['value'] = number_format($position['stockPrice'] * $position['shares'], 2, '.', '');
		}
	}
	unset($position);
}

function addMonthlyShares(&$positions) {
	foreach ($positions as $symbol => &$position) {
		if (isset($position['addMonthly']) && $position['addMonthly'] > 0) {
			$position['shares'] += $position['addMonthly'];
			$position['value'] = number_format($position['stockPrice'] * $position['shares'], 2, '.', '');
		}
	}
	unset($position);
}

echo "Starting positions (Symbol -> Shares -> Value):" . PHP_EOL;
	foreach ($positions as $symbol => $position) {
		echo "\t{$symbol} \t\t {$position['shares']} \t\t {$position['value']}" . PHP_EOL;
	}
	echo "Portfolio Value: " . array_sum(array_column($positions, 'value')) . PHP_EOL;
	echo PHP_EOL;

$year = 1;
$years = 10;
$months = 12;
while($year <= $years){
	for($i = 1; $i <= $months; $i++){
		monthlyDividends($positions);
		if ($i % 3 == 0) {
			quarterlyDividends($positions);
		}
		addMonthlyShares($positions);
	}
	
	echo "Positions after year #$year (Symbol -> Shares -> Value):" . PHP_EOL;
	foreach ($positions as $symbol => $position) {
		echo "\t{$symbol} \t\t {$position['shares']} \t\t {$position['value']}" . PHP_EOL;
	}
	echo "Portfolio Value: " . array_sum(array_column($positions, 'value')) . PHP_EOL;
	echo PHP_EOL;
	$year++;
}

Run this like php estimate.php and the end result outputs