php生成XML怎么加BOM

        $dom = new DOMDocument("1.0","utf-8"); 
        header("Content-Type: text/html; charset=unicode");
        //items
        $root = $dom->createElement("items"); 
        $dom->appendChild($root); 
        //count
        $count = $dom->createAttribute("count"); 
        $root->appendChild($count); 
        $count_value = $dom->createTextNode("6"); 
        $count->appendChild($count_value); 
        
        //item
        $item = $dom->createElement("item"); 
        $root->appendChild($item); 
        $text = $dom->createTextNode($content);

        $item->appendChild($text); 
        //id        
        $id = $dom->createAttribute("id"); 
        $item->appendChild($id); 
        $idValue = $dom->createTextNode("156"); 
        $id->appendChild($idValue); 
        //title
        $title = $dom->createAttribute("title"); 
        $item->appendChild($title); 
        $titleValue = $dom->createTextNode("test"); 
        $title->appendChild($titleValue); 
        $content=$dom->saveXML();