Adds months to a date without skipping february

add-months-to-date.php

<?php 
/* As inspired by http://stackoverflow.com/a/10734891/3156582 */
$date = '2017-01-31';
$nbremois = -13;
$d1 = DateTime::createFromFormat('Y-m-d', $date);
    
    $year = $d1->format('Y');
    $month = $d1->format('m');
    $day = $d1->format('d');
    
    $year += $nbremois >= 0 ? floor($nbremois / 12) : ceil($nbremois/12);
    //$year += floor($nbremois / 12);
    $nbremois = $nbremois % 12;
    $month += $nbremois;
    //echo $month%12 + 12;
    
    if( $month > 12 ) {
        $year ++;
        $month = $month % 12;
        if( $month === 0 ) {
            $month = 12;
        }
    }
    if( $month < 0 ) {
        $month = 12 + $month % 12;
        $year--;
    }
    
    if( !checkdate($month, $day, $year) ) {
        $d2 = DateTime::createFromFormat('Y-m-d', $year . '-' . $month . '-1');
        $d2->modify('last day of');
    } else {
        $d2 = DateTime::createFromFormat('Y-m-d', $year . '-' . $month . '-' . $day);
    }
    
    echo $d2->format('Y-m-d');