cast month and day to prevent white screens on My reservation tab using php8

reserve.inc

@@ -1091,7 +1091,7 @@ function reserve_which_year($month, $day) {
   // determine if this year or next year
   $yearnow = date('Y');
   $absdaynow = date('z');
-  $absdaydefault = date('z', mktime(0, 0, 0, $month, $day, $yearnow));
+  $absdaydefault = date('z', mktime(0, 0, 0, (int)$month, (int)$day, $yearnow));
   if ($absdaynow > $absdaydefault) {
     $year = $yearnow + 1;
   }