ช่วยแปลความหมายของ code ให้หน่อยคับ อยากได้ลายละเอียดหน่อย เพราะไม่เข้าใจ

//Today's date if $start_date is a Sunday, otherwise date of previous Sunday
$today_or_previous_sunday = mktime(0, 0, 0, date('m', $start_date), date('d', $start_date), date('Y', $start_date)) - ((date("w", $start_date) ==0) ? 0 : (172800*date("w", $start_date)));

//date_default_timezone_set('America/Denver'); //Set apprpriate timezone
date_default_timezone_set('UTC');    


//echo $date_1;
$start_date = strtotime('2011-05-01'); //Set start date   

//Today's date if $start_date is a Sunday, otherwise date of previous Sunday
$today_or_previous_sunday = mktime(0, 0, 0, date('m', $start_date), date('d', $start_date), date('Y', $start_date)) - ((date("w", $start_date) ==0) ? 0 : (172800*date("w", $start_date)));

//prints 12-13-2009 (month-day-year)
echo date('d-m-Y', $today_or_previous_sunday);