prorate

upgrade.php

<?php

// dummy data
$start = new DateTime('-9 month');
$expiry = new DateTime("+3 month");
$package_a = 399;
$package_b = 699;

// expiry day does not count thus the substrction of one day
$days = ($expiry->diff($start)->format("%a")) - 1;
$remaining_days = ($expiry->diff(new DateTime("NOW"))->format("%a")) - 1;

// $payable containes the amount that need to be paid
$payable = round($package_b - (
			$package_a - (
				($package_a / $days) * ($days - $remaining_days))
			) , 2);