Calculate a proration percentage given the current timestamp and a current billing period

stripe-proration.php

<?php

/**
 * Handle calculating a percentage/fraction (proration) we should charge the
 * user for based on the current day of the month before their next bill cycle.
 * To use yourself, implement a getSubscription method which returns an object
 * containing current_period_start and current_period_end DateTime objects.
 *
 * @access  public
 * @return  float
 */
function prorateUpcomingBillingCycle()
{
    $now = new DateTime('now', new DateTimeZone('UTC'));

    // determine the next billing cycle
    $subscription       = getSubscription();
    $currentPeriodStart = $subscription->current_period_start;
    $currentPeriodEnd   = $subscription->current_period_end;

    if (is_string($currentPeriodStart)) {
        $currentPeriodStart = new DateTime($currentPeriodStart, new DateTimeZone('UTC'));
    }

    if (is_string($currentPeriodEnd)) {
        $currentPeriodEnd = new DateTime($currentPeriodEnd, new DateTimeZone('UTC'));
    }

    // get the number of second difference between the cycle start and end date
    $currentPeriodStartEpoch    = (int) $currentPeriodStartEpoch->format('U');
    $currentPeriodEndEpoch      = (int) $currentPeriodEndEpoch->format('U');
    $nowEpoch                   = (int) $now->format('U');

    // if we aren't between the start and end of the subscription period, we have a problem
    if ($nowEpoch < $currentPeriodStartEpoch || $nowEpoch > $currentPeriodEndEpoch) {
        throw new Exception('The current timestamp does not fall within the current subscription period.');
    }

    // get the difference of the start and end time in seconds
    $epochDifference = $currentPeriodEndEpoch - $currentPeriodStartEpoch;

    // get the prorated number of seconds till the end of the subscription period
    $remainingSecondsInPeriod = $currentPeriodEndEpoch - $nowEpoch;

    // return fraction of the total seconds in the current billing period
    return $remainingSecondsInPeriod / $epochDifference;
}

/**
 * Returns an example subscription object with a 30 day subscription period.
 * 
 * This is only for reference.
 * 
 * @return  object
 */
function getSubScription() 
{
    $now = new DateTime('now', new DateTimeZone('UTC'));
    
    $obj = new stdClass();
    $obj->current_period_start = clone $now;
    $obj->current_period_start->sub(new DateInterval('P15D');
    $obj->current_period_end = clone $now;
    $obj->current_period_end->add(new DateInterval('P15D')));
    
    return $obj;
}

Utilize this method if you don't want to tie yourself down to a third parties proration handler in all cases and chose to do so on your own accord. It can handle any billing cycle lengths (monthly, bi-monthly, quarterly, yearly) as it's based on seconds since the epoch and the start and end dates of the billing cycle.

Thank you!