Where there are funky characters in the XML file,DOMDocument->load complained about them, saying that they weren’t UTF-8, and to specify the encoding.

For example,

Warning: DOMDocument::loadXML(): Input is not proper UTF-8, indicate encoding !
Bytes: 0xE4 0x3C 0x2F 0x66 in Entity, line: 1 in test.php on line 6
with doc->recover=false(default) : failed
Solution：

<?php

$doc = new DOMDocument('1.0', 'iso-8859-1');
$doc->load($xmlPath);
?>

Edit: Since it appears that this doesn't work, what you could do instead is similar to your existing method but without the temp file. Read the XML file from your source just using standard IO operations (file_get_contents() or something), then perform whatever changes to the encoding you need (iconv() or utf8_decode()) and then use loadXML()
<?php
 
$myXMLString = file_get_contents($xmlPath);
$myXMLString = utf8_decode($myXMLString);
$doc = new DOMDocument('1.0', 'iso-8859-1');
$doc->loadXML($myXMLString);
?>

For example:
<?php
function getNodeValue($dom, $tagName, $nodeName)

 {

  $notificationNodeList = $dom->getElementsByTagName($tagName);

  if (($notificationNodeList->length == 1) && $notificationNodeList->item(0)->hasChildNodes())

  {

   foreach($notificationNodeList->item(0)->childNodes as $node)

   {

    if ($node->nodeName == $nodeName)

     return trim($node->nodeValue);

   }

  }

  return "";

 }
$xmlStr = utf8_decode($xmlStr);
$dom = new DomDocument('1.0', 'UTF-8');

 $dom->loadXml($xmlStr);
 $firstname = getNodeValue($dom, 'LicenseeContact', 'cbt:Firstname');

 $lastname  = getNodeValue($dom, 'LicenseeContact', 'cbt:Lastname');
 foreach($dom->getElementsBytagName('Item') as $items):

     $m_item[] = $items->getElementsByTagName('ProductId')->item(0)->firstChild->nodeValue;

     $powerNum_Quantity[] = $items->getElementsByTagName('Quantity')->item(0)->firstChild->nodeValue;

     $YourCurrencyId[]=$items->getElementsByTagName('YourCurrencyId')->item(0)->firstChild->nodeValue;

     endforeach;
?>