gistfile1.php

$array = array(
	'Response' => array(
		'OneKey' => 'foo',
		'TwoKey' => array('bar', 'baz')
	)
);
		
echo $xml->header();
echo $xml->serialize($array, array('format' => 'tags'));

gistfile2.xml

<response>
<OneKey>foo</OneKey>
<two_key>bar</two_key>
<two_key>baz</two_key>
</response>