使用soap时，无法连接到主机错误

I am using SOAP to call a web servicefrom a Linux Centos 6 server and a php client. In this week I have been getting could not connect to host error from soapCall method. My code is as below and I have not changed it at all for some months but recently it gets this error most of the time. I have read most answers to related questions here but my problem have not been solved.

$wsdl="http://x.x.x.x:x/gw/services/Service?wsdl";

//Set key as HTTP Header 
$aHTTP['http']['header'] =  "key:" .$key ."
";

$context = stream_context_create($aHTTP);

try
{
  $client = new SoapClient($wsdl,array("soap_version" => SOAP_1_2,'trace' => 1,"stream_context" => $context));
}
catch(Exception $e)
{
        return "something";             
}
//I make $parametrs
try
{

 $res = $client->__soapCall("send",array($parametrs));
}
catch(Exception $e)
{
    print_r($e->getMessage()); //Most of the time it prints could not connect to host
}

I changed SOAP from _1_1 to _1_2 but nothing changed.

This is how I call SOAP webservice, please note Service?wsdl should be Service.wsdl

Example

//Initialize values
$wsdl = "Service.wsdl";
$url = "http://x.x.x.x:x/gw/services/";
$username = "********"; //add username
$password = "********"; //add password


$client = new SoapClient("$url".$wsdl);
$params = array(
                "username"=>$username,
                "password"=>$password
                );
$response = $client->UserLogIn($params); //UserLogIn is the function name

var_dump($response);  // to see webservice response