$data = [
            'email'     => 'johndoe@example.com',
            'status'    => 'subscribed',
            'firstname' => 'john',
            'lastname'  => 'doe'
        ];

        syncMailchimp($data);

        function syncMailchimp($data) {
            $apiKey = 'Tha api key';
            $listId = 'The lis id';

            $memberId = md5(strtolower($data['email']));
            $dataCenter = substr($apiKey,strpos($apiKey,'-')+1);
            $url = 'https://' . $dataCenter . '.api.mailchimp.com/3.0/lists/' . $listId . '/members/' . $memberId;

            $json = json_encode([
                'email_address' => $data['email'],
                'status'        => $data['status'], // "subscribed","unsubscribed","cleaned","pending"
                'merge_fields'  => [
                    'FNAME'     => $data['firstname'],
                    'LNAME'     => $data['lastname']
                ]
            ]);

            $ch = curl_init($url);

            curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);
            curl_setopt($ch, CURLOPT_HTTPHEADER, ['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);

            return $httpCode;
        }