在尝试向xml文档添加新节点时出现错误.使用DOM和PHP

我想创建属性并将其添加到xml元素。我总是得到错误的文档错误，当我试图添加任何属性或子元素到$commentElement。错误发生在带有注释的行://Error。下面是我的代码:

if (empty($_POST['userName']))
    printErrorMessage("Username is not filled in");
if (empty($_POST['commentArea']))
    printErrorMessage("Comment is not filled in");
$username = mysql_real_escape_string($_POST['userName']);
$comment = mysql_real_escape_string($_POST['commentArea']);
$newsId = $_GET['newsId'];
$fileContainingNew = $_GET['fileContainingNew'];
$xmlDoc = new DOMDocument('1.0', 'windows-1257');
$root = NULL;
$commentId = NULL;
$commentElement = $xmlDoc->createElement("komentaras");
if (!file_exists(substr($fileContainingNew, 0, strlen($fileContainingNew) - 4) . "Comments.xml")) {
    $root = $xmlDoc->createElement("naujienuKomentarai");
    $xmlDoc->appendChild($root);
    $commentId = 1;
}
else {
    $xmlDoc->load(substr($fileContainingNew, 0, strlen($fileContainingNew) - 4) . "Comments.xml");
    $commentsList = $xmlDoc->getElementsByTagName("komentaras");
    if ($commentsList->length == 0)
        $commentId = 1;
    else {
        $biggestCommentId = 0;
        foreach ($commentsList as $comment){
            if ($newsId == $comment->getAttributeNode("newId")->value){
                if ($comment->getAttributeNode("commentId")->value > $biggestCommentId)
                    $biggestCommentId = $comment->getAttributeNode("commentId")->value; 
            }
        }
        $commentId = $biggestCommentId++;
    }
    $root = $xmlDoc->documentElement;
}
$root->appendChild($commentElement);//error 
$commentElement = appendAttribute($xmlDoc, $commentElement, "newId", $newsId);
$commentElement = appendAttribute($xmlDoc, $commentElement, "commentId", $commentId);
$commentElement = appendElement($xmlDoc, $commentElement, "autorius", $username);
$commentElement = appendElement($xmlDoc, $commentElement, "data", '20' . date("y-m-d"));
$commentElement = appendElement($xmlDoc, $commentElement, "laikas", "no val");
$commentElement = appendElement($xmlDoc, $commentElement, "ip", $_SERVER['REMOTE_ADDR']);
$commentElement = appendElement($xmlDoc, $commentElement, "komentaroTekstas", $comment);
$xmlDoc->formatOutput = true;
$xmlDoc->save(substr($fileContainingNew, 0, strlen($fileContainingNew) - 4) . "Comments.xml");
function appendAttribute($xmlDoc, $parentElement, $attributeName, $newAttributeValue){
    $attribute = $xmlDoc->createAttribute($attributeName);
    $attributeValue = $xmlDoc->createTextNode($newAttributeValue);
    $parentElement->appendChild($attribute);
    $attribute->appendChild($attributeValue);
    return $parentElement;
}
function appendElement($xmlDoc, $parentElement, $newElementName, $newElementValue){
    $newElement = $xmlDoc->createElement($newElementName);
    $elementValue = $xmlDoc->createTextNode($newElementValue);
    $newElement->appendChild($elementValue);
    $parentElement->appendChild($newElement);
    return $parentElement;
}

帮忙吗?