<?php

 date_default_timezone_set('Europe/Amsterdam');
 $date = new DateTime(); 

 $ano=2018;
 $semana=47;
 
 echo $date->setISODate($ano, $semana)->format('d-m-Y');

 echo PHP_EOL;
     
 echo $date->setISODate($ano, $semana, 7)->format('d-m-Y');