- str_replace: documentation ( source)
<?php
$datetime1 = new DateTime();
$datetime2 = new DateTime('2013-01-03 17:13:00');
$interval = $datetime1->diff($datetime2);
$elapsed = $interval->format('%a days %h hours %i minutes %S seconds');
$empties = array('0 days', '0 hours', '0 minutes', '0 seconds');
echo str_replace($empties, '', $elapsed);