<?php

date_default_timezone_set('Europe/Amsterdam');

$date = new DateTime();

$date->modify('first day of this week');
echo 'First day of this week: '.$date->format('Y-m-d')."\n";

$date->modify('first day of week 50');
echo 'First day of week 50: '.$date->format('Y-m-d')."\n";

$date->modify('last day of this week');
echo 'Last day of this week: '.$date->format('Y-m-d')."\n";

$date->modify('last day of week 50');
echo 'Last day of week 50: '.$date->format('Y-m-d')."\n";

First day of this week: 2011-12-18
First day of week 50: 2011-12-19
Last day of this week: 2011-12-18
Last day of week 50: 2011-12-17