$time = strtotime("2010.12.11");
$final = date("Y-m-d", strtotime("+1 month", $time));
$future_timestamp = strtotime("+1 month");
$data = date('Y-m-d', $future_timestamp);
<?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));
?>