Modify xml with php [Post]

Hi i want to modify an xml with php in a server. I hace the n

<?php

 $idFiesta = $_POST['idFiesta'];
 $comentario = $_POST['comentario'];

$doc = new DOMDocument();
$doc->preserveWhiteSpace = false;
$doc->load( 'comentarios.xml' );
$xpath = new DOMXPath($doc);

$doc->formatOutput = true;
$query="//[@idFiesta=$idFiesta]";

$element = $xpath->query($query);
$nodo =$element->item(0);
$nodo.appendChild($doc->createTextNode( $comment[$comentario]));

$doc->save('comentarios.xml');
?>

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<Comentarios>
    <Comentario idFiesta="1">
                <coment>me gusto mucho</coment>
        </Comentario>
</Comentarios>

It was a post message, i wat to insert a comment if the id is the same. I have an error in the query and in the appendChild() I thik. Thanks!