php - Formula to get payment dates of direct debit bills -

having billing payment method bills set direct debit, , installment periods may vary: 30/60/90 days, 30/45/60, 15/30/45 , on, how can accurately installment dates?

this have far, doesn't work instance 30/45/60, or 15/45/75:

$dates = [];       $invoicedate = explode('-', $invoicedate); // yyyy-mm-dd   generation date   foreach ($installments $i => $installment) {  // e.g.: $installment = [30, 45, 60]      if ($installment % 30 == 0) {         $month = round($installment / 30);          $date = mktime(0, 0, 0, $invoicedate[1] + $month, $invoicedate[2], $invoicedate[0]);                         }     else {          $month = 0;         if ($installment > 30 && count($installments) > 1) {             $month = floor($installment / 30);             $installment = abs($installment - ($month * 30));         }          $monthoffset = 30 - date('t', mktime(0, 0, 0, $invoicedate[1] + $month, 1, date('y')));          $date = mktime(0, 0, 0, $invoicedate[1] + $month, (int)$invoicedate[2] + (int)$installment - $monthoffset, $invoicedate[0]);      }         $dates[] = date('y-m-d', $date); } 

with code, if had bill on 2016-04-15, payments 30/60/90 work, returning these payment dates:

15/05/2016 15/06/2016 15/07/2016

however, not payments 15/45/75:

30/04/2016 31/05/2016 (should 30) 30/06/2016

nor 30/45/60:

15/05/2016 31/05/2016 (should 30) 15/06/2016

if billing done in 2016-04-30, payment works: 30/45/60

30/05/2016 15/06/2016 30/06/2016

so 15/45/75:

15/05/2016 15/06/2016 15/07/2016

and 30/60/90:

30/05/2016 30/06/2016 30/07/2016

any idea how implement algorithm takes account possibilities?

use datetime object manipulate date

 $periods = [ [30,60,90],             [30,45,60],             [15,30,45]]; $date =  '2016-04-15';  $d = new datetime($date);  foreach ($periods $ps)     foreach($ps $p) {         $t = new datetime($d->format('y-m-d'));           echo $t->modify('+'.$p.' days')->format('y-m-d')."\n";         unset($t);    } 

result

2016-05-15 2016-06-14 2016-07-14 2016-05-15 2016-05-30 2016-06-14 2016-04-30 2016-05-15 2016-05-30