laboral_days.php

<?php
  function laboral_days($start, $end, $nowork = array('sun', 'sat'), $festives = array()){
    $days = array('sat' => 5, 'sun' => 6);
    $time_start = strtotime($start);
    $time_end = strtotime($end);
    $day_init = date('w', $time_start);
    $day_end = date('w', $time_end);

    $total_days = ($time_end - $time_start) / (60 * 60 * 24) + 1;
    $weeks = floor($total_days / 7);
    $efective_days = $total_days - $weeks * count($nowork);

    if(($day_end - $day_init) != 0){
      foreach($nowork as $d){
        $d = $days[$d];
        if($d > $day_init && $d <= $day_end){
          $efective_days--;
        }
      }
    }

    foreach($festives as $d){
      $e_t = strtotime($d);
      if($e_t >= $time_start && $e_t <= $time_end){
          $efective_days--;
      }
    }
    return $efective_days;
  }

echo laboral_days('17-04-2013', '30-04-2013');