Lettura DOM

[starcraftworld]

$xml=fread($fp,filesize("file.xml"));
    $dom->loadXML($xml);
    $root=$dom->documentElement;
    if ($nodeengine=$root->getElementsByTagName("engine")&&$nodeactive=$root->getElementsByTagName("active"))
        if ($nodeengine->item(0)->nodeValue=="on"&&$nodeactive->item(0)->nodeValue=="off")
            echo "OK"; 

Produce l'errore:

Call to a member function item() on a non-object