如何在PHP中获取SOAP服务器函数名？

I want to get soap server function name to put array of variable and send it but the problem is i don't have exact name of function and i looking for a method to test it so i did this :

$options = array('username' => $this->user, 'password' => $this->pass);
$client = new SoapClient($this->wsdl_link, $options);

And then

print_r($client->__getTypes());

it return :

Array
(
    [0] => struct SendSms {
 string username;
 string password;
 string from;
 ArrayOfString to;
 string text;
 boolean flash;
}
    [1] => struct ArrayOfString {
 string string;
}
    [2] => struct SendSmsResponse {
 ArrayOfString SendSmsResult;
}
    [3] => struct Credit {
 string username;
 string password;
}
    [4] => struct CreditResponse {
 double CreditResult;
}
    [5] => struct Delivery {
 string recId;
}
    [6] => struct DeliveryResponse {
 int DeliveryResult;
}
    [7] => struct MultiDelivery {
 ArrayOfString recId;
}
    [8] => struct MultiDeliveryResponse {
 ArrayOfInt MultiDeliveryResult;
}
    [9] => struct ArrayOfInt {
 int int;
}
    [10] => struct GetVirtualNumbers {
 string username;
 string password;
}
    [11] => struct GetVirtualNumbersResponse {
 ArrayOfString GetVirtualNumbersResult;
}
    [12] => struct GetPhoneGroups {
 string username;
 string password;
}
    [13] => struct GetPhoneGroupsResponse {
 ArrayOfString GetPhoneGroupsResult;
}
    [14] => struct SendToGroup {
 string username;
 string password;
 string from;
 ArrayOfInt groupIds;
 string text;
 boolean flash;
}
    [15] => struct SendToGroupResponse {
 int SendToGroupResult;
}
    [16] => struct ScheduledSend {
 string username;
 string password;
 string from;
 ArrayOfString to;
 ArrayOfInt groupIds;
 string text;
 boolean flash;
 int year;
 int month;
 int day;
 int hour;
 int minute;
}
    [17] => struct ScheduledSendResponse {
 int ScheduledSendResult;
}
    [18] => struct GetScheduleStatus {
 string username;
 string password;
 int scheduleId;
}
    [19] => struct GetScheduleStatusResponse {
 int GetScheduleStatusResult;
}
    [20] => struct RemoveScheduledSend {
 string username;
 string password;
 int scheduleId;
}
    [21] => struct RemoveScheduledSendResponse {
 int RemoveScheduledSendResult;
}
    [22] => struct GetInbox {
 string username;
 string password;
 string to;
 int year;
 int month;
 int day;
 int hour;
 int minute;
}
    [23] => struct GetInboxResponse {
 ArrayOfString GetInboxResult;
}
)

but when i type

$credit = $client->Credit();

i get :

soap:Server Server was unable to process request. ---> Object reference not set to an instance of an object. soap:Server Server was unable to process request. ---> Object reference not set to an instance of an object.

What should i do to get function name?

Edited: Output of __getFunctions:

        Array
(
    [0] => SendSmsResponse SendSms(SendSms $parameters)
    [1] => CreditResponse Credit(Credit $parameters)
    [2] => DeliveryResponse Delivery(Delivery $parameters)
    [3] => MultiDeliveryResponse MultiDelivery(MultiDelivery $parameters)
    [4] => GetVirtualNumbersResponse GetVirtualNumbers(GetVirtualNumbers $parameters)
    [5] => GetPhoneGroupsResponse GetPhoneGroups(GetPhoneGroups $parameters)
    [6] => SendToGroupResponse SendToGroup(SendToGroup $parameters)
    [7] => ScheduledSendResponse ScheduledSend(ScheduledSend $parameters)
    [8] => GetScheduleStatusResponse GetScheduleStatus(GetScheduleStatus $parameters)
    [9] => RemoveScheduledSendResponse RemoveScheduledSend(RemoveScheduledSend $parameters)
    [10] => GetInboxResponse GetInbox(GetInbox $parameters)
    [11] => SendSmsResponse SendSms(SendSms $parameters)
    [12] => CreditResponse Credit(Credit $parameters)
    [13] => DeliveryResponse Delivery(Delivery $parameters)
    [14] => MultiDeliveryResponse MultiDelivery(MultiDelivery $parameters)
    [15] => GetVirtualNumbersResponse GetVirtualNumbers(GetVirtualNumbers $parameters)
    [16] => GetPhoneGroupsResponse GetPhoneGroups(GetPhoneGroups $parameters)
    [17] => SendToGroupResponse SendToGroup(SendToGroup $parameters)
    [18] => ScheduledSendResponse ScheduledSend(ScheduledSend $parameters)
    [19] => GetScheduleStatusResponse GetScheduleStatus(GetScheduleStatus $parameters)
    [20] => RemoveScheduledSendResponse RemoveScheduledSend(RemoveScheduledSend $parameters)
    [21] => GetInboxResponse GetInbox(GetInbox $parameters)
)