日付～DateTimeクラス～

<?php
$date = DateTime::createFromFormat('j-M-Y', '15-Feb-2009');
echo $date->format('Y-m-d');
?>

今日

$nowtime = new DateTime("now");

$yesterday = new DateTime("now");
$yesterday->sub(new DateInterval('P1D'));

$now = $nowtime->format('Y-m-d H:i:s');