+1 month is working but not date php? code example

Example 1: in date function + 1 month and - 1 day in php

echo Date("Y-m-d", strtotime("2013-01-01 +1 Month -1 Day"));// 2013-01-31

echo Date("Y-m-d", strtotime("2013-01-31 +1 Month -3 Day")); // 2013-02-28

echo Date("Y-m-d", strtotime("2013-01-31 +2 Month")); // 2013-03-31

echo Date("Y-m-d", strtotime("2013-01-31 +3 Month -1 Day")); // 2013-04-30

echo Date("Y-m-d", strtotime("2013-12-31 -1 Month -1 Day")); // 2013-11-30

echo Date("Y-m-d", strtotime("2013-12-31 -2 Month")); // 2013-10-31

echo Date("Y-m-d", strtotime("2013-12-31 -3 Month")); // 2013-10-01

echo Date("Y-m-d", strtotime("2013-12-31 -3 Month -1 Day")); // 2013-09-30

Example 2: in date function + 1 month and - 1 day in php

<?php
$months = n; // Here n = …..-2,-1,0,1,2, …..(months to add or subtract)
$years = n;  // Here n = …..-2,-1,0,1,2, …..(years to add or subtract)
echo date('Y-m-28', mktime(0, 0, 0, date('m')+$months, 1, date('Y') + $years));
?>