<?php
$date = new DateTimeImmutable( '2018-01-31 15:48:21' );
echo $date->modify( 'next month' )->format( 'Y-m-d' ), "\n";
?>
Output
2018-03-03
Explanation:
2018-01-31 15:48:21
2018-02-31 15:48:21
"next month" increases month number by 1
2018-03-03 15:48:21
February only has 28 days, so days are overflown
into the next month