What can go wrong when adding months with a DateInterval and DateTime::add?

Question

I failed to find a proper solution to this issue. As you see in Example #3 in the PHP documentation, they state that one must beware when adding months using the DateInterva

Answer 1

If your goal is to strictly increment by user-friendly months (thus, 3 months from January 21st should be April 21st), with the exception that shorter membership months get shortened (thus, 1 month from January 31st is February 28th/29th), then you only need to go back a few days if you crossed over into the next month:

function addMonths($date,$months) {
  $orig_day = $date->format("d");
  $date->modify("+".$months." months");
  while ($date->format("d")<$orig_day && $date->format("d")<5)
    $date->modify("-1 day");
}

$d = new DateTime("2000-01-31");
addMonths($d,1);
echo $d->format("Y-m-d"); // 2000-02-29