Normal Doctrinized Invoice

invoice.php

<?php

class Invoice {
  private $_items;
  private $_totalAmount = 0;
  private $_sentDate;

  public function isSent() {
    return !is_null($this->_sentDate);
  }

  public function removeItem() {
    if ($this->isSent()) {
      throw new Exception('Items cannot be changed after invoice is sent');
    }

    $this->_totalAmount -= $this->_items[$index];
    unset($this->_items[$index]);
  }

}