If the target month is smaller than the original month, the extra days will be discarded.
/**
* Shift a giving months from your input datetime with month based
*
* If the target month is smaller than the original month, the extra days will be discarded.
*
* @author Nick Tsai <myintaer@gmail.com>
* @example
* shiftMonths("2018-01-15", '+', 1); // 2018-02-15
* shiftMonths("2018-01-31", '+', 1); // 2018-02-28
* shiftMonths("2020-02-29", "-", 12); // 2019-02-28
* shiftMonths("2018-01-15", '+', 1, false); // 1518681600
* shiftMonths("2020-02-29 10:11:12", "-", 24, "Y-m-d H:i:s"); // 2018-02-28 10:11:12
*
* @param string Datetime string
* @param string Operation: +/-
* @param integer Number of months
* @param string PHP date() format, empty returns Unix time
* @return string|integer Date format|Unix time
*/
function shiftMonths($date, $operation='+', $months=1, $outputFormat="Y-m-d")
{
$timestamp = strtotime($date);
// Year
$op = ($operation=='+') ? '+' : '-';
$newBasetime = strtotime(date("Y-m-1 H:i:s", $timestamp) . " {$op}{$months} month");
// Month
$m = date("m", $newBasetime);
// Day
$lastDay = date("t", $newBasetime);
$day = date("d", $timestamp);
$day = ($lastDay <= $day) ? $lastDay : $day;
$result = strtotime(date("Y-m-{$day} H:i:s", $newBasetime));
return ($outputFormat) ? date($outputFormat, $result) : $result;
}