try the following code...
<?php
$date_str = "2002-04-01 10:00:00";
$date_val = strtotime($date_str);
$date_array = getdate($date_val);
$yy = $date_array["year"];
$mm = $date_array["mon"];
$dd = $date_array["mday"];
$hh = $date_array["hours"];
$mi = $date_array["minutes"];
$ss = $date_array["seconds"];
$prev_mth_last_day = mktime($hh, $mi, $ss, $mm, ($dd - 1), $yy);
print date("d M, Y H:i:s", $prev_mth_last_day);
?>