DOM appenchild at wrong position

I have a problem with appendChild in a XML file. I wrote a little script to edit a XML file. You can change the value of a node or add a new one.

for($i=0;$i<count($value);$i++){
$element = $xpath->query('//parent/child[index="'.$index.'"]//'.$key)->item($i);

if($element){
    $element->nodeValue = $value[$i];
} else {
    $elementLast = $xpath->query('//parent/child[index="'.$index.'"]//'.$key)->item($i-1);
    $elementLast->appendChild($dom_object->createElement($key, $value[$i]));
}}

The new node is created but it looks like this

<child>Blablabla<child>new node</child></child>

So the new node is inserted in the last child node not after it like it should be.