date and timestamp in php

we went through this topic in todays php class

<?php
echo date("Y/m/d") . "<br>";
echo date("Y.m.d") . "<br>";
echo date("Y-m-d");
?>

and

<?php
$tomorrow = mktime(0,0,0,date("m"),date("d")+1,date("Y"));
echo "Tomorrow is ".date("Y/m/d", $tomorrow);
?>

the output is:

Tomorrow is 2009/05/12