OK, its not a function, but it gives you an idea of what you can do. First get the day of week for the date in question, then subtract that from the date - and there's your answer.
To get the last day in week, just add 6 to the first day in week.
<?php
$date_param = "25 dec 2000";
$dstamp = strtotime($date_param);
$day = date("w",$date_stamp);
$first_in_week = mktime(0,0,0,date("m",$dstamp),date("d",$dstamp)-$day,date("Y",$dstamp));
echo date("d m Y",$first_in_week);
?>
