$oldDate = new DateTime('first day of September last year');
$nowDate = new DateTime();
$oldYear = $oldDate->format('Y');
$nowYear = $nowDate->format('Y');
$maxOldDay = new DateTime($oldYear.'-12-31');
$maxOldWeek = intval($maxOldDay->format('W'));
$oldWeek = intval($oldDate->format('W'));
$nowWeek = intval($nowDate->format('W'));
$diff = 0;
if ($nowWeek > $oldWeek){
$diff = $oldWeek - $nowWeek + 1;
} elseif ($nowWeek < $oldWeek){
$diff = $nowWeek + ($maxOldWeek - $oldWeek) + 1;
} else $diff = 1;
$this->sendMessage($user_id, 'сейчас = '.$diff.' неделя');