First you would write a function like this. Note that this function treats sunday as the first day of the week and not monday as in your post, but that could be fixed pretty easily.
<?php
function first_day_of_week($date = '') {
if($date == '') $date = time();
$day = date('w',$date);
//the number of seconds since the most recent sunday.
$offset = $day * 86400;
$start_of_week = $date - $offset;
return $start_of_week;
} //end first_day_of_week
?>
The you would call it like this
<?php
$time = first_day_of_week();
echo "Today is " . date('d-m-Y') . " Sunday was " . date('d-m-Y',$time);
?>