PHPの組み込みの日付関数を使用します：
<?php
$start_time = "Y-m-d H:i:s"; // fill this in with actual time in this format
$end_time = "Y-m-d H:i:s"; // fill this in with actual time in this format
// both of the above formats are the same as what MySQL stores its
// DATETIMEs in
$start = new DateTime($start_time);
$interval = $start->diff(new DateTime($end_time));
echo $interval->format("d \d\a\y\s h \h\o\u\r\s");