Unable to save the last xml file php - php

Question

I am fairly new to php, and I have written code to work with the amazon API. When I request information from the API, I receive it, but am unable to sort through the XML. Here is the error:
Fatal error: Call to a member function children() on null in J:\XAMPP\htdocs\Phillip\src\MarketplaceWebServiceProducts\Samples\csv_prep.php on line 117
Here is the code:
if(is_array($xmlFiles)){
foreach($xmlFiles as $xmlFile){
$xml = simplexml_load_file($xmlFile);
foreach($xml->GetMatchingProductForIdResult as $items) {
//Line 117 ->
if(isset($items->Products->Product->AttributeSets->children('ns2', true)->ItemAttributes->ListPrice->Amount) !== False) {
$amount = $items->Products->Product->AttributeSets->children('ns2', true)->ItemAttributes->ListPrice->Amount
}else{
$amount = '0.00';
}
}
}
}
The tag in the XML that I am trying to get the value of looks like this:
<ns2:amount>9.99</ns2:amount>
It is in the same place as it says in the code. I only have this problem with large files and I am not sure what is happening. If someone could help, I would greatly appreciate it. Thanks in advance!

Answer 1

I'm not certain, but based on the error message, it looks like somewhere in the $items->Products->Product->AttributeSets hierarchy something you're specifying does not exist.
Do tests to see if $items or $items->Products or $items->Products->Product or $items->Products->Product->AttributeSets and whichever one fails, print out that fact and call exit;.
You could do a print_r on $items to help as well.