MailChimp API在V3.0 PHP Curl中响应404

I am using the following code to initiate a member add request to a list in MailChimp.

<?php

error_reporting(-1);
    ini_set('display_errors', 1);

$email = 'test@domain.com';
$first_name = 'name';
$last_name = 'last name';

$api_key = 'xx-us18'; // YOUR API KEY

// server name followed by a dot. 
// We use us13 because us13 is present in API KEY
$server = 'us18.'; 

$list_id = 'xx'; // YOUR LIST ID

$auth = base64_encode( 'user:'.$api_key );

$data = array(
    'apikey'        => $api_key,
    'email_address' => $email,
    'status'        => 'subscribed',
    'merge_fields'  => array(
        'FNAME' => $first_name,
        'LNAME'    => $last_name
        )    
    );
$json_data = json_encode($data);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://'.$server.'api.mailchimp.com/3.0/lists/'.$list_id.'/members');
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json',
    'Authorization: Basic '.$auth));
curl_setopt($ch, CURLOPT_USERAGENT, 'PHP-MCAPI/2.0');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 10);
curl_setopt($ch, CURLOPT_POST, true);    
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, $json_data);

$result = curl_exec($ch);

echo $result;
?>

But what happened is the response is 404.

{
type: "http://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/",
title: "Resource Not Found",
status: 404,
detail: "The requested resource could not be found.",
instance: "78a35efa-ef43-471d-9e70-aecdc992d2e6"
}

Does anyone know what I am doing wrong? What might be the reason for this error?

Extra Infos: API Key is valid, If I provide that wrong, I am getting another error. List-id is also true. I am running this code on localhost with MAMP.

The issue was I used web_id instead of list_id. When I changed that, the issue was resolved.