<?php
$thismonth = date('m');
$thisyear  = date('Y');
$startDay = $thisyear . '-' . $thismonth . '-1';
$endDay   = $thisyear . '-' . $thismonth . '-' . date('t', strtotime($startDay));
$b_time       = strtotime($startDay);
$e_time       = strtotime($endDay);

<?php
$thismonth = date('m');
$thisyear  = date('Y');
if ($thismonth == 1) {
    $lastmonth = 12;
    $lastyear  = $thisyear - 1;
} else {
    $lastmonth = $thismonth - 1;
    $lastyear  = $thisyear;
}
$lastStartDay = $lastyear . '-' . $lastmonth . '-1';
$lastEndDay   = $lastyear . '-' . $lastmonth . '-' . date('t', strtotime($lastStartDay));
$b_time = strtotime($lastStartDay);
$e_time = strtotime($lastEndDay);