<?php

function end_date ($start_date, $working_days) {
  $modulo = $working_days%5;
  $weekday = date ('N', $start_date);
  $days=7*(int)($working_days/5)+$modulo+max(0,$modulo?$weekday-3:0);
  return $start_date + $days*60*60*24;
}

$start_date = date_timestamp_get(date_create ('2014-01-20'));
for ($i=0; $i < 6; ++$i) {
   echo date ('d/m/Y',end_date($start_date,$i)), PHP_EOL;
}