php日期月份解决办法

$first_day_of_month = "2013-12";
 $t = strtotime($first_day_of_month);
 print_r(array(
             date('Y-m',$t),
             date('Y-m',strtotime('+ 1 month',$t))
 ));

echo date('Y-m', strtotime('2013-12-01 +1 month'));