Wordpress Membermouse Getting Data for Mailchimp - php

Question

I am working on a MemberMouse subscription Wordpress website. I want to get data of the user into the php file via API to a Mailchimp mailing list.
I wrote the following script, that should be called under the condition that once the script is loaded it sends the following php sript:
<?php
require_once 'MCAPI.class.php';
$apikey='Your-API-Key'; // Enter your API key
$api = new MCAPI($apikey);
$retval = $api->lists();
$listid='List-Id'; // Enter list Id here
$uname = $_GET["username"]; // Enter subscriber last name
$email = $_GET["email"]; // Enter subscriber email address
$merge_vars = array('FNAME' => $uname);
if($api->listSubscribe($listid, $email,$merge_vars) === true) {
echo 'success';
}
?>
However, it always gives me an error and can not read the user data.
Here is the Membermouse Wordpress Hooks: http://support.membermouse.com/support/solutions/articles/9000020294-membermouse-wordpress-hooks#member-data
And here if using PHP interface: https://membermouse.uservoice.com/knowledgebase/articles/319071-using-the-php-interface
The MCAPI.class.php is another php that reads data for Mailchimp. It is simply the MailChimp API PHP Class, which I integrated it into my project.
By the way, manually adding the member via the script above is no problem:
<?php
require_once 'MCAPI.class.php';
$apikey='Your-API-Key'; // Enter your API key
$api = new MCAPI($apikey);
$retval = $api->lists();
$listid='List-Id'; // Enter list Id here
$email='Subscriber-email-address'; // Enter subscriber email address
$name='Subscriber-first-name'; // Enter subscriber first name
// By default this sends a confirmation email - you will not see new members
// until the link contained in it is clicked!
$merge_vars = array('FNAME' => $name);
if($api->listSubscribe($listid, $email,$merge_vars) === true) {
echo 'success';
}
?>