- floor: documentation ( source)
- ceil: documentation ( source)
<?php
$minutes = 5;
echo (new DateTime('2001-01-01 01:' . (floor($dt->format('i')/$minutes) * $minutes) . ':60'))->format('Y-m-d H:i:s') . "\n";
echo (new DateTime('2001-01-01 01:' . (ceil($dt->format('i')/$minutes) * $minutes) . ':60'))->format('Y-m-d H:i:s') . "\n";