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