Thanks @dootzky & @lulco. I have solved this. Code below works perfectly fine for me:
<?php
ini_set("soap.wsdl_cache_enabled", "0"); // disabling WSDL cache
$wsdl_path = "http://www.xxxxxxx/zzzzzzzzzz/service.asmx?WSDL";
$login_id = 'XXXX';
$password = 'YYYYYY';
$client = new SoapClient($wsdl_path, array('trace' => 1));
try {
echo "<pre>\n";
$result = $client->SOME_FUNCTION(array("request" => array("cedula" => $login_id, "contrasena" => $password)))
print_r($result);
echo "\n";
}
catch (SoapFault $exception) {
echo $exception;
}
?>