In the docs for google sitemaps it is required an element for mobile sitemaps that looks like this: <mobile:mobile/>
I used some time to figure out how to make it, but it is quite simple when understood.
$mobile_schema = 'http://www.google.com/schemas/sitemap-mobile/1.0';
//Create root element
$xml_mobile = new SimpleXMLElement('
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:mobile="'.$mobile_schema.'"></urlset>
');
//Add required children
$url_mobile = $xml_b_list_mobile->addChild('url');
$url_mobile->addChild('loc', 'your-mobile-site-url');
$url_mobile->addChild('mobile:mobile', null, $mobile_schema);
For this to work properly the attribute xmlns:mobile must be set in the root node, and then used as namespace(third argument) when creating the mobile:mobile child with null as value.