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