# xmlns="" attribute This helper is simply container for XML namespaces definitions used by your document. Like many other helpers from this bundle, it is extension to [`ArrayObject`](http://php.net/manual/en/class.arrayobject.php). It aggregates all namespaces as `namespace => alias` map: ```php ``` You can set default XML namespace by setting empty value for namespace: ```php ``` When namespace is registered you can then use helper also to produce prefix for elements with `getPrefix()` method: ```php <getPrefix('http://ogp.me/ns/fb#'); ?>like/> ``` Which should produce: ```html ``` To output all defined `xmlns` attributes simply call: ```php > … ``` Output has leading space so you don't have to leave extra space between `html` element name and helper output.