The conversion to a SimpleXML Object and the output works, see code example below. Check your "products.xml" file for the correct UTF-8 encoding type.
<?php
$xml = <<< XML
<?xml version="1.0" encoding="UTF-8"?>
<result>
<orderlist>
<order_no>123123</order_no>
<date></date>
<client_name>Knapp & Donut Shop</client_name>
<sector>54</sector>
</orderlist>
</result>
XML;
$xml = str_replace(array("&", "&"), array("&", "&"), $xml);
$xmlObj = simplexml_load_string($xml);
var_dump($xmlObj);
echo PHP_EOL . $xmlObj->orderlist->client_name;
// Result: Knapp & Donut Shop