Can't add user to a group using MailChimp API 3.0 - php

Question

I'm trying to update my code for subscribing new users to my MailChimp newsletter.
I would like it to add the user to a group, so that I can distinguish between users when sending out newsletters. This code, however, subscribes the user but does not add it to any group. What am I doing wrong? I am using the MailChimp API 3.0.
My code:
$apiKey = 'XXXX';
$list_id = 'XXXX';
$memberId = md5(strtolower($data['email']));
$dataCenter = substr($apiKey,strpos($apiKey,'-')+1);
$url = 'https://' . $dataCenter . '.api.mailchimp.com/3.0/lists/' . $list_id . '/members/' . $memberId;
$json = json_encode(array(
'email_address' => $data['email'],
'status' => $data['status'], // "subscribed","unsubscribed","cleaned","pending"
'merge_fields' => array(
'FNAME' => $data['firstname'],
'LNAME' => $data['lastname'],
'GROUPINGS' => array(
0 => array(
'name' => 'Interests',
'groups' => array( $data['tag'] ),
),
),
),
));
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_TIMEOUT, 10);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POSTFIELDS, $json);
$result = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

Answer 1

You are passing Interest grouping in the wrong array.
you need to pass the interest group id in interests array with value as true.
$json = json_encode(array(
'email_address' => 'test#example.com',
'status' => 'subscribed',
'merge_fields' => array(
'FNAME' => 'FirstName',
'LNAME' => 'LastName',
),
'interests' => array(
'9143cf3bd1' => true,
'789cf3bds1' => true
),
));
You will get the id of interest groups [9143cf3bd1,789cf3bds1 in above example] of the list by requesting to this URL
/lists/{list_id}/interest-categories
see doc here
If you want to remove the subscriber from the group, you need to set its value to false in your update request.