The following code works for me while it appears soap.amazon.com has been deprecated and taken out of service. http://webservices.amazon.com has replaced Amazon's SOAP.
<?php
$client = new SoapClient('http://webservices.amazon.com/AWSECommerceService/AWSECommerceService.wsdl');
var_dump($client->__getFunctions());
?>