<?php
 
define ('ONE_DAY', 60*60*24);
function end_date ($start_date, $working_days) {
  $weekday = date ('N', $start_date)-1;
  if ($weekday > 4) {
  	$start_date += ONE_DAY*(7-$weekday);
  	$weekday=0;
  }
  $modulo = $working_days%5;
  $days=7*(int)($working_days/5)+$modulo;
  $eweekday = $weekday + $modulo;
  if ($eweekday > 4)
    $days += max(2,7-$eweekday);
  return $start_date + ONE_DAY*$days;
}
 
$start_date = date_timestamp_get(date_create ('2014-01-04'));
for ($i=10; $i < 15; ++$i) {
  echo date ('d/m/Y',end_date($start_date,$i)), PHP_EOL;
}

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