$weekStart	= date('Y-m-d H:i:s', strtotime(date('Y').'W'.date('W').'1'));
$weekEnd	= date('Y-m-d H:i:s', strtotime(date('Y').'W'.date('W').'7'));

// или так

$current_week_start = date("d.m.Y", strtotime("last Monday"));
$current_week_end = date("d.m.Y", strtotime("Sunday"));

// для любого timestamp $firstWeek;

$firstWeekStart= date(("d.m.Y" , strtotime("Monday",$firstWeek));
$firstWeekEnd= date(("d.m.Y" , strtotime("Sunday",$firstWeek));