SoapClient LibXMLError:无法加载外部实体，SOAP-ERROR:解析WSDL:无法加载

我正在尝试使用以下PHP代码建立SOAP连接，但在SoapClient构造时失败了：

// Need to declare these settings here because our php.ini has alternate
// settings due to global purposes for other PHP scripts
ini_set("soap.wsdl_cache_enabled", "0");
ini_set("soap.wsdl_cache", "0");
ini_set("display_errors","On");
ini_set("track_errors","On");
// FedEx web services URL, note the HTTPS
$path_to_wsdl = 'https://wsbeta.fedex.com/web-services';
$soap_args = array(
    'exceptions'=>true,
    'cache_wsdl'=>WSDL_CACHE_NONE,
    'trace'=>1)
;
try {
    $client = new SoapClient($path_to_wsdl,$soap_args);
} catch (SoapFault $e) {
    var_dump(libxml_get_last_error());
    echo "<BR><BR>";
    var_dump($e);
}

这输出：

object(LibXMLError)#1 (6) {
    ["level"]=> int(1)
    ["code"]=> int(1549)
    ["column"]=> int(0)
    ["message"]=> string(71) "failed to load external entity "https://wsbeta.fedex.com/web-services" "
    ["file"]=> string(0) ""
    ["line"]=> int(0)
}
object(SoapFault)#2 (9) {
    ["message":protected]=> string(158) "SOAP-ERROR: Parsing WSDL: Couldn't load from 'https://wsbeta.fedex.com/web-services' : failed to load external entity "https://wsbeta.fedex.com/web-services" "
    ["string":"Exception":private]=> string(0) ""
    ["code":protected]=> int(0)
    ["file":protected]=> string(53) "/mnt/array/bell-enterprise/bell/fedex_shipservice.php"
    ["line":protected]=> int(34)
    ["trace":"Exception":private]=> array(1) {
        [0]=> array(6) {
            ["file"]=> string(53) "/mnt/array/bell-enterprise/bell/fedex_shipservice.php"
            ["line"]=> int(34)
            ["function"]=> string(10) "SoapClient"
            ["class"]=> string(10) "SoapClient"
            ["type"]=> string(2) "->"
            ["args"]=> array(2) {
                [0]=> string(37) "https://wsbeta.fedex.com/web-services"
                [1]=> array(4) {
                    ["exceptions"]=> bool(true)
                    ["soap_version"]=> int(1)
                    ["cache_wsdl"]=> int(0)
                    ["trace"]=> int(1)
                }
            }
        }
    }
    ["previous":"Exception":private]=> NULL
    ["faultstring"]=> string(158) "SOAP-ERROR: Parsing WSDL: Couldn't load from 'https://wsbeta.fedex.com/web-services' : failed to load external entity "https://wsbeta.fedex.com/web-services" "
    ["faultcode"]=> string(4) "WSDL"
}