callback.php

<?php

public function checkauth() {
    $config = $this->config('snsso.settings');
    $authCode = null;
    $user = null;

    if (isset($_GET['code'])) {
        if ($_GET['code']) $authCode = $_GET['code'];
    }

    if ($authCode) {
        $checkUrl = \Drupal::request()->getSchemeAndHttpHost() . Url::fromRoute('snsso.checkauth')->toString();
        
        // Build token URL
        $params = array(
            'grant_type' => 'authorization_code',
            'code' => $authCode,
            'redirect_uri' => $checkUrl,
            'client_id' => $config->get('client_credentials.client_id'),
            'client_secret' => $config->get('client_credentials.client_secret'),
        );
        $tokenUrl = Url::fromUri($config->get('sso_server_endpoints.token_endpoint'), ['query' => $params])->toString();

        // di CI ganti ini dengan curl
        $client = \Drupal::httpClient();
        $response = $client->request('POST', $tokenUrl,[
            'http_errors' => false,
        ]);
        // /. di CI ganti ini dengan curl
        
        $statusCode = $response->getStatusCode();
        if ($statusCode == 200) {
            // get access token
            $json = json_decode($response->getBody()->getContents(), true);
            $accessToken = $json['access_token'];

            if ($accessToken) {
                // user info url
                $userInfoUrl = Url::fromUri($config->get('sso_server_endpoints.userinfo_endpoint'))->toString();
                $response = $client->request('GET', $userInfoUrl,[
                    'http_errors' => false,
                    'headers' => [
                        'Authorization' => 'Bearer ' . $accessToken,
                    ],
                ]);
                
                $statusCode = $response->getStatusCode();
                if ($statusCode == 200) {
                    // get user info
                    $json = json_decode($response->getBody()->getContents(), true);

                    /** your code
                    *
                    * --- CODE ---
                    *
                    * end of your code */
                }
            }
        }
    }
}