gistfile1.php

<?php  
$things = array (
  'customer',
	'invoicetype',
	'invoicenumber',
	'status',
	'currency',
	'period',
	'invoicedate',
	'duedate',
	'bank',
	'invoiceaddressnumber',
	'deliveraddressnumber',
	'headertext',
	'footertext'
);
	
foreach ($things as $thing) {
	${$thing}Node    = $this->createTextNode($invoice->get{$thing}());
	${$thing}Element = $this->createElement($thing);
	${$thing}Element->appendChild({$thing}Node);
	$headerElement->appendChild(${$thing}Element);
}