final_value_with_tax.php

<?php

$now = 100.00;
$months = 12;
$monthly = 100.00;
$tax = 6.5;

if ($monthly) {
  $result = ($monthly * (((1 + $tax / 100) ** $months) - 1) / ($tax / 100));
} else {
  $result = ($now * (((1 + $tax / 100) ** $months)));
}

$profit = $result - $months * $monthly;

echo "You will have: " . number_format($result, 2);
echo "\n";
echo "Profit of: " . number_format($profit, 2);