PHP SoapClient - 调用未定义的函数

I have this code:

<?php
$login = '';
$password = '';
$session = new SoapClient("https://mybilling.telinta.com/wsdl/SessionAccountService.wsdl");

echo '<pre>';
print_r($session->__getFunctions());
$session_id = $session−>login($login, $password);

And this gives me this output:

Array
(
    [0] => int ping(string $session_id)
    [1] => string logout(string $session_id)
    [2] => string login(string $user, string $password)
)


Fatal error:  Call to undefined function login() in C:\.... on line 14

Why error is produced ?