How to use date() function in php

<?php
//backdate
$backdate= strtotime("-1 week");
echo date("d-m-y\n",$backdate);
//next date
$backdate= strtotime("+1 week");
echo date("d-m-y\n",$backdate);
//back 1 year
$backdate= strtotime("-1 year");
echo date("d-m-y\n",$backdate);
//next two week
$backdate= strtotime("+3 years");
echo date("d-m-y\n",$backdate);
?>