<?php

// trait Tax
trait Tax
{
  protected function calculate($value, $days)
  {
    return $value * .2 + (.03 * $days * $value);
  }
}

trait Date
{
  protected function diff()
  {
    return ;// todo: diff in days between two dates
  }

  protected function today()
  {
    return date('Y-m-d'); // todo: something better then this
  }
}

// class Bill
class Bill extends Model
{
  use Tax;

  function save($data)
  {
    $data->tax = $this->calculate($data->value, $this->diff($data->payment, $this->today()));

    parent::save($data);
  }
}