无法加载外部实体

我正在尝试集成联邦快递 API 并收到此错误。

警告：SoapClient：：SoapClient（） [soapclient.soapclient]：I/O 警告：无法加载外部实体"../wsdl/RateService_v13.wsdl" 在测试中.php在第 12 行

致命错误： 未捕获的 Soap错误异常： [WSDL] 肥皂错误： 解析 WSDL： 无法从"../wsdl/RateService_v13.wsdl' in D：''wamp''www''fedexapi''AddressValidationService_v4_php''php''AddressValidationWebServiceClient''rate_test.php：12 堆栈跟踪： #0 D：''wamp''www''fedexapi''AddressValidationService_v4_php''php''AddressValidationWebServiceClient''rate_test.php（12）： SoapClient->SoapClient（'../wsdl/RateSer...'， Array） #1 {main} 在测试中抛出.php在第 12 行

<?php
// Copyright 2009, FedEx Corporation. All rights reserved.
// Version 2.0.0
require_once('../library/fedex-common.php5');
//The WSDL is not included with the sample code.
//Please include and reference in $path_to_wsdl variable.
$path_to_wsdl = "../wsdl/LocationsService_v3.wsdl";
ini_set("soap.wsdl_cache_enabled", "0");
$client = new SoapClient($path_to_wsdl, array('trace' => 1)); // Refer to http://us3.php.net/manual/en/ref.soap.php for more information
$request['WebAuthenticationDetail'] = array(
        'ParentCredential' => array(
            'Key' => getProperty('parentkey'),
            'Password' => getProperty('parentpassword')
        ),
        'UserCredential' => array(
            'Key' => getProperty('key'), 
            'Password' => getProperty('password')
    )
);
?>

如何解决这个问题？