(PHP 5, PHP 7, PHP 8)
DOMNode::lookupNamespaceURI — Gets the namespace URI of the node based on the prefix
   Gets the namespace URI of the node based on the 
   prefix.
  
prefix
       The prefix to look for. If this parameter is null, the method will return the default namespace URI, if any.
      
   Returns the associated namespace URI or null if none is found.