PHP DateAdd

<?php
date('Y-m-d', strtotime($time))
date('Y-m-d', mktime($int-time))
?>

<?php
echo date('Y-m-d',strtotime('+1 month'))."<br>";
echo date('Y-m-d',strtotime("now"))."<br>";
echo date('Y-m-d',strtotime("10 September 2000"))."<br>";
echo date('Y-m-d',strtotime("+1 day"))."<br>";
echo date('Y-m-d',strtotime("+1 week"))."<br>";
echo date('Y-m-d',strtotime("+1 week 2 days 4 hours 2 seconds"))."<br>";
echo date('Y-m-d',strtotime("next Thursday"))."<br>";
echo date('Y-m-d',strtotime("last Monday"))."<br>";
echo date("Y-m-d H:i:s", mktime(date("H"), date("i")+0, date("s")+0, date("m")+0  , date("d")+0, date("Y")+0))."<br>";
?>

$strStartDate = "2013-04-21";
$strNewDate = date ("Y-m-d", strtotime("+3 day", strtotime($strStartDate)));