I'm also getting same error while passing correct parameter. Below are the code.
$functionname = 'core_enrol_get_enrolled_users';
$params = array('courseid' => 64, 'options' => array());
// SOAP CALL
$serverurl = $domainname . '/webservice/soap/server.php'. '?wsdl=1&wstoken=' . $token;
////Do the main soap call
$client = new SoapClient($serverurl);
try {
$resp = $client->__soapCall($functionname, array($params));
} catch (Exception $e) {
print_r($e);
}
if (isset($resp)) {
print_r($resp);
}
Thank you.