如何使用项目（索引）删除子节点？

I'm writing a script that search in multiple XML files for some tag and then if it's find in this tag child named update I need to delete that child and then add it again.

Problem is that I don't understand why it doesn't deletes nodes I want to delete.

Ok so my script (important part I want to discuss) looks like this:

/*
// Pushing all offers from all files to $allOffers array
*/
foreach ($offerFiles as $file)
{
    $file = $path . "\\" . $file;
    $currentXML = new SimpleXMLElement($file, 0, true);
    foreach($currentXML->offer as $offer)
    {
        if ($offer->number) {
            if (!check_if_exists($compiledXML, $offer->number))
            { 
               //array_push($allOffers, $offer); 
            } 
            if (check_if_exists($compiledXML, $offer->number) && $offer->action == "update")
            {
                update_existing_entry($compiledFile, $compiledXML, $offer); 

                // var_dump($allOffers);

            }
        }
    }

}

/*
// Find and delete existing XML entry offer with update action
*/

function update_existing_entry ($compiledFile, $compiledXML, $parsedOffer) {


    $index = 0;
    $doc = new DOMDocument();
    $doc->load($compiledFile);
    $elem = $doc->documentElement;


    foreach ($compiledXML->offer as $offer) {  
        if ((string)$parsedOffer->number === (string)$offer->number) {

            $firstchild =  $doc->getElementsByTagName('offer')->item($index);

            // $firstchild->nodeValue = null;
            $elem->removeChild($firstchild);
            $doc->save($compiledFile);

          //var_dump($parsedOffer->asXML());   
        }
        $index++;

    }
    var_dump($deleteNodes);
}

Now if I have 2 XML files, 1 with update action, another without it then it works perfect. Problems starts when 1 and 2 files has update action, then I always ends with only one deleted node and error:

Fatal error: Uncaught TypeError: Argument 1 passed to DOMNode::removeChild() must be an instance of DOMNode, null given

Why I can't delete nodes with selected index?

I don't know if it's the best approach, but I have fixed it this way:

   function update_existing_entry ($compiledFile, $compiledXML, $parsedOffer) {


    $doc = new DOMDocument();
    $doc->load($compiledFile);
    $node = $doc->documentElement;

    foreach ($doc->getElementsByTagName('offer') as $child) {
        if (strpos($child->nodeValue, (string)$parsedOffer->number) !== false) {
            $node->removeChild($child);
        }

    }
    $doc->save($compiledFile);

}