Get the timestamp for the beginning of the last quarter.

last_quarter_timestamp.php

<?php

$start_date = strtotime('3 months ago');
$start_quarter = ceil(date('m', $start_date) / 3);
$start_month = ($start_quarter * 3) - 2;
$start_year = date('Y', $start_date);
$start_timestamp = mktime(0, 0, 0, $start_month, 1, $start_year);

echo $start_timestamp;

last_quarter_timestamp.sql

SELECT UNIX_TIMESTAMP(STR_TO_DATE(CONCAT('1,', ((QUARTER(DATE_SUB(NOW(), INTERVAL 3 MONTH)) * 3) - 2), ',', YEAR(NOW())),'%d,%m,%Y')) 

great idea.
i needed an end date of the quarter for a range.

if anybody is in need, try this:
$end_timestamp = strtotime('last day of '.date('F', strtotime("+2 month",strtotime(date('F',$start_timestamp)."1")))." {$start_year}");