MonthlyPlan.php

<?php

class MonthlyPlan
{
	public int $length = 1;
	public string $unit = 'month';
	
	public function normalizeBillingPeriod(Subscription $subscription, Carbon $charge_at): array
	{
		// First, we'll naively compute the end of the billing period (assuming the timing is 100% accurate)

		$period_ends_at = $charge_at->add(unit: $this->unit, value: $this->length, overflow: false);
		
		// Next, we get the rounded number of months from the start of the subscription to the date of the charge 

		$months_elapsed = round($subscription->starts_at->floatDiffInMonths($charge_at));
		
		// Now we need the billing period length in months. Because plans can be on any schedule, we
		// will compute this dynamically with rounding to account for any leap year rounding issues

		$period_months = round($charge_at->floatDiffInMonths($period_ends_at));
		
		// Finally, we can compute the actual range of the billing period
        
		$period_starts_at = $subscription->starts_at->addMonthsNoOverflow($months_elapsed);
		$period_ends_at = $subscription->starts_at->addMonthsNoOverflow($months_elapsed + $period_months);
		
		return [$period_starts_at, $period_ends_at];
	}
}