<?php
$year = 2015;
$period = new DatePeriod(
new DateTime($year.'-01-01'),
new DateInterval('P1M'),
new DateTime(($year+1).'-01-01')
);
foreach($period as $date){
$firstDay = $date->modify('first day of this month')->format("Y/m/d") ;
$lastDay = $date->modify('last day of this month')->format("Y/m/d") ;
echo "$firstDay - $lastDay \n";
}