<?php
$start = new DateTime('2013-04-25');
//$start->modify('first day of this month');
$end = new DateTime('2014-04-25');
//$end->modify('first day of next month');
$interval = DateInterval::createFromDateString('1 month');
$period = new DatePeriod($start, $interval, $end);
foreach ($period as $dt) {
echo $dt->format("m-Y") . PHP_EOL;
}