DomNode->attributes() problem

Hi

I wan't to get attribute values for a tags "<kaup>". I used functions:
DomNode->attributes()
DomElement->get_attribute("nr")

and they both worked, but it gives me an error for using those functions:
Fatal error: Call to a member function on a non-object in ... line of 
funct...

Tell me what to have to do to get rid of that error.

xml file "tooted.xml":

<?xml version="1.0" encoding="ISO-8859-1"?>
<tooted>
<kaup nr="1">
         <mudel>SH07ZPGA</mudel>
         <firma>samsung</firma>
</kaup>
<kaup nr="2">
         <mudel>ACH1800E</mudel>
         <firma>samsung</firma>
</kaup>
</tooted>

here is php code:

<?php
class Toode{

     function Toode(){
         $dom = domxml_open_file("tooted.xml");
         $root = $dom->document_element();
         $kaubad=$root->get_elements_by_tagname("kaup");

         for($i=0; i<count($kaubad); $i++){
          	$k=$kaubad[$i];
             $nr=$k->attributes();
             	echo "kauba id: ".$nr[0]->value()."<br>";

         }
     }
}

$test=new Toode();

?>


and with get_attributes("nr"):

<?php
class Toode{

     function Toode(){
         $dom = domxml_open_file("tooted.xml");
         $root = $dom->document_element();
         $kaubad=$root->get_elements_by_tagname("kaup");

         for($i=0; i<count($kaubad); $i++){
          	$k=$kaubad[$i];
             $nr=$k->get_attribute("nr");
             	echo "kauba id: ".$nr."<br>";

         }
     }
}

$test=new Toode();

?>