(PHP 5, PHP 7)
DOMNode::lookupNamespaceUri — Gets the namespace URI of the node based on the prefix
$prefix
) : string
Gets the namespace URI of the node based on the
prefix.
prefixThe prefix of the namespace.
The namespace URI of the node.
You have to pass *null* to lookupNamespaceURI if you want to read the default namespace. Passing an empty string doesn't work - even though the documentation says that lookupNamespaceURI takes a string as argument (and null would just be converted to an empty string).
Simple way to get the URI of the default namespace:
<?php
$document = new DOMDocument();
$document->load($xml_file);
$uri = $document->documentElement->lookupnamespaceURI(NULL);
?>