First day of previous month, last day of previous month using DATETIME class. Requires PHP 5.3 to work ("first day of" is introduced in PHP 5.3).
<?php
$month_ini = new DateTime("first day of last month");
$month_end = new DateTime("last day of last month");
echo $month_ini->format('Y-m-d'); // 2012-02-01
echo "<br/>";
echo $month_end->format('Y-m-d'); // 2012-02-29
echo "<br/>";
?>