PHP determine remaining percent of a month - php

Question

I need to apply discounts to our billing services and am having a devil of a time trying to figure out how to determine the used percent of an applied discount. I have scrapped almost 10 versions of code and think I am just to close to the problem and am not able to see this clearly.
I am trying to make universal function that will use the first/last day of the billing cycle, plus the startDiscount and endDiscount dates to determine what percent to apply to the actual discount; regardless if the discount is a % discount or an actual $ value.
Example: Bill month March 2023 (31 day bill cycle), invoiced services from Mar 1st - Mar 31st. But a 50% discount was applied starting on March 10th - the discount is for 90 days.
This means that roughly 66% of the 50% discount (IE: 33%) needs to be applied to the march invoice, the full 50% of April and May...and then 34% of the 50% discount (IE: 17%) needs to be applied to Junes bill.
So for the applied discounts:
March = 66% of the 50% discount = 33% discount applied to charges
April = 100% of the 50% discount = 50% discount applied to the charges
May = = 100% of the 50% discount = 50% discount applied to the charges
June = 33% of the 50% discount = 17% discount applied to charges
So as each invoice is processed, the first/last day of the invoice range is passed in as well as the start/finish dates of the discount
$discPercent = applyDiscount("2023-03-01","2023-03-31","2023-03-10","2023-06-10") ;
$discPercent = applyDiscount("2023-04-01","2023-04-30","2023-03-10","2023-06-10") ;
$discPercent = applyDiscount("2023-05-01","2023-05-31","2023-03-10","2023-06-10") ;
$discPercent = applyDiscount("2023-06-01","2023-06-30","2023-03-10","2023-06-10") ;
function applyDiscount($firstDay,$lastDay,$startDisc,$endDisc) {
...
...
return $applyThisPercent ;
}
Then $discPercent can be applied an existing 90 day 50% discount OR or can be applied to $100 credit per invoice cycle for 90s

Answer 1

You need to calculate the number of days overlapping the billing period and the discount period and divide it by number of days in the billing period.
With the help of the overlap function in this answer, you get this:
function datesOverlap($start_one, $end_one, $start_two, $end_two) {
if($start_one <= $end_two && $end_one >= $start_two) { //If the dates overlap
return datesDays(min($end_one, $end_two), max($start_two, $start_one));
}
return 0; //Return 0 if there is no overlap
}
function datesDays($start, $end) {
return $start->diff($end)->days+1;
}
function applyDiscount($firstDay, $lastDay, $startDisc, $endDisc) {
$firstDay = new DateTime($firstDay);
$lastDay = new DateTime($lastDay);
$startDisc = new DateTime($startDisc);
$endDisc = new DateTime($endDisc);
$all_days = datesDays($firstDay, $lastDay);
$used_days = datesOverlap($firstDay, $lastDay, $startDisc, $endDisc);
return round(100*$used_days/$all_days);
}
echo applyDiscount("2023-03-01","2023-03-31","2023-03-10","2023-06-10")."\n";
echo applyDiscount("2023-04-01","2023-04-30","2023-03-10","2023-06-10")."\n";
echo applyDiscount("2023-05-01","2023-05-31","2023-03-10","2023-06-10")."\n";
echo applyDiscount("2023-06-01","2023-06-30","2023-03-10","2023-06-10")."\n";
This results in:
71
100
100
33