Just in case it isn't clear (like I had), an example:

<?php

$domDocument = new DOMDocument('1.0', "UTF-8");
$domElement = $domDocument->createElement('field','some random data');
$domAttribute = $domDocument->createAttribute('name');

// Value for the created attribute
$domAttribute->value = 'attributevalue';

// Don't forget to append it to the element
$domElement->appendChild($domAttribute);

// Append it to the document itself
$domDocument->appendChild($domElement);

?>

Will output:
<?xml version="1.0" encoding="UTF-8"?>
<field name="attributevalue">some random data</field>