今月の月初
echo date("Y-m-01", time());
今月の月末
echo date("Y-m-t", time());
前月の月初
/* echo date("Y-m-01",strtotime("-1 month")); */
訂正
$now = strtotime("2013-07-31");
echo date("Y-m-d",strtotime("first day of - 1 month",$now));
前月の月末
/* echo date("Y-m-t",strtotime("-1 month")); */
訂正
$now = strtotime("2013-07-31");
echo date("Y-m-d",strtotime("last day of - 1 month",$now));