how to get the first and last days of a given month

You might want to look at the strtotime and date functions.

<?php

$query_date = '2010-02-04';

// First day of the month.
echo date('Y-m-01', strtotime($query_date));

// Last day of the month.
echo date('Y-m-t', strtotime($query_date));

Try this , if you are using PHP 5.3+, in php

$query_date = '2010-02-04';
$date = new DateTime($query_date);
//First day of month
$date->modify('first day of this month');
$firstday= $date->format('Y-m-d');
//Last day of month
$date->modify('last day of this month');
$lastday= $date->format('Y-m-d');

For finding next month last date, modify as follows,

 $date->modify('last day of 1 month');
 echo $date->format('Y-m-d');

and so on..

I know this question has a good answer with 't', but thought I would add another solution.

$first = date("Y-m-d", strtotime("first day of this month"));
$last = date("Y-m-d", strtotime("last day of this month"));