If you want to find the first day and last day from the specified date variable then you can do this like below:
$date = '2012-02-12';//your given date
$first_date_find = strtotime(date("Y-m-d", strtotime($date)) . ", first day of this month");
echo $first_date = date("Y-m-d",$first_date_find);
$last_date_find = strtotime(date("Y-m-d", strtotime($date)) . ", last day of this month");
echo $last_date = date("Y-m-d",$last_date_find);
For the current date just simple use this
$first_date = date('Y-m-d',strtotime('first day of this month'));
$last_date = date('Y-m-d',strtotime('last day of this month'));