- var_dump: documentation ( source)
<?php
$start = '2014-03-20 18:23:15';
$day="Tuesday";
$start=DateTime::createFromFormat('Y-m-d H:i:s', $start);
$nextOccurence = clone $start;
$nextOccurence->modify("next $day")->setTime((int)$start->format('H'), (int)$start->format('i'), (int)$start->format('s'));
var_dump($nextOccurence);