function getDateTime($now, $term=1) {
     $time = strtotime(($term>0?"+":"-").$term." month", strtotime(date("Y-m",strtotime($now))." -01 "));
     $l = date('t', $time);
     return date("Y-m-".$l, $time);
}



foreach(array('2015-10-31','2015-11-30','2015-12-31','2016-01-31','2016-02-29','2016-03-31','2016-04-30','2016-05-31','2015-10-04','2015-11-18','2015-12-27') as $date ){
     echo $date.' → '.getDateTime($date,3).'<br>';
}



결과물
2015-10-31 → 2016-01-31
2015-11-30 → 2016-02-29
2015-12-31 → 2016-03-31
2016-01-31 → 2016-04-30
2016-02-29 → 2016-05-31
2016-03-31 → 2016-06-30
2016-04-30 → 2016-07-31
2016-05-31 → 2016-08-31
2015-10-04 → 2016-01-31
2015-11-18 → 2016-02-29
2015-12-27 → 2016-03-31