public function getChildPIDs($process_count = null)
  { 
    $child_pids = array();
    $try = true;
    
    while ($try == true)
    {
      if (file_exists($this->working_directory."pids"))
      {
        $ct = file_get_contents($this->working_directory."pids");
        $child_pids = preg_split("#\n#", $ct, -1, PREG_SPLIT_NO_EMPTY);
        
        if ($process_count == null) $try = false;
        if (count($child_pids) == $process_count) $try = false;
      }
      
      sleep(0.2);
    }
    
    return $child_pids;
    
  }