Add P1M can skip next month in some #PHP cases

If you are using new DateInterval('P1M') for adding month to date to get next month, do not do that. There is huge difference between adding to begin and end of the month. It does sense, but it is against semblance at first sight of P1M.

$begin = new DateTime('2019-03-01');
$begin->add(new DateInterval('P1M'));
echo $begin->format('Y-m'); // 2019-04

$end = new DateTime('2019-03-31');
$end->add(new DateInterval('P1M'));
echo $end->format('Y-m'); // 2019-05

How to get next month

$date = new DateTime('2019-03-31');
$date->modify('first day of next month');
echo $date->format('Y-m'); // 2019-04

The first day of next month is longer string than P1M, but it’s self-explanatory and works every time as you expect.