MyConnection.php

<?php

$xml = file_get_contents('myxmlfile.xml');

$url = "http://webservices.test.aspx";

$post_data = array(
	'xml' => $xml,
);

$stream_options = array(
	'http' => array(
		'method' =>'POST',
		'header'=> "content-type: application/xml",
		'content' => http_build_query($post_data),
	)
);

$context  = stream_context_create($stream_options);
$response = file_get_contents($url, false, $context);


print_r($response);

?>

myxmlfile.xml

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="GetItemInfo"
		elementFormDefault="qualified"
		xmlns:xs="http://www.w3.org/2001/XMLSchema"
>
<GetItemInfo>
<Authentication>
<Username>Test1</Username>
<Password>test123</Password>
<OrderID>4528412158352483213215</OrderID>
</Authentication>
<ItemNumber>Itemnumber123</ItemNumber>
<Manufacturer>Bosch</Manufacturer>
<Origin>10</Origin>
<Quantity>10</Quantity>
</GetItemInfo>