The class DOMNode doesn't provide a contructor and also no static methods like createInstance(). So, how to create an DOMNode instance?
Create a DOMDocument. It extends from DOMNode, has a constructor, and represents the rootnode of a document. Being a DOMNode, DOMDocument has methods for adding children. For instance, DOMDocument has a method CreateElement, which returns a DOMElement, which also inherits from DOMNode. All in all, it seems that DOMNode is just the base class and shouldn't be used directly.