- substr: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$Start_Week = '201338';
$Optel_Week = 8;
$Year_numbers= substr($Start_Week, 0, 4);
$Week_numbers= substr($Start_Week, 4, 6);
$Start_Week_berekend = date("Y-m-d", strtotime("{$Year_numbers}-W{$Week_numbers} + $Optel_Week week"));
echo $Start_Week_berekend;