Вы можете использовать strtotime () s this week
селектор.
<?php
$ts_today = strtotime('2011-12-14');
$ts_firstDayOfWeek = strtotime('this week', $ts_today);
echo 'today: ', date('Y-m-d', $ts_today), "\n";
echo 'week start: ', date('Y-m-d', $ts_firstDayOfWeek), "\n";
печать
today: 2011-12-14
week start: 2011-12-12