<?php
  $date = new DateTimeImmutable( '2018-01-31 15:48:21' );
  echo $date->modify( 'first day of next month' )->format( 'Y-m-d' ), "\n";
?>
Output
2018-02-01

Explanation:

2018-01-31 15:48:21

2018-02-31 15:48:21 "next month" increases month number by 1

2018-02-01 15:48:21 "first day of" resets the day number to 1