<?php

define ('ONE_DAY', 60*60*24);
function end_date ($start_date, $working_days) {
  $weekday = date ('N', $start_date)-1;
  if ($weekday > 4) {
  	$start_date -= ONE_DAY*($weekday-4);
  	$weekday=4;
  }
  $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;
}

for ($j=0; $j < 7; ++$j) {
  $start_date = date_timestamp_get(date_create ('2013-12-30'))+ONE_DAY*$j;
  for ($i=10; $i < 15; ++$i)
    echo date ('d/m/Y',$start_date),' : ',date ('d/m/Y',end_date($start_date,$i)), PHP_EOL;
}