xdazz's answer is almost correct, but you will absorb the current time if you do that without removing it explicitly.
<?
$dt = DateTime::createFromFormat("m/d/Y", "05/04/2014");
$dt->setTime(0,0,0); # remove current time.
echo $dt->getTimestamp();
?>
In understand why we need line 2, replace line 3 with
var_dump($dt);
Then, delete line 2, and observe that there is a time in addition to the desired date.