API license key plugin verification. Visit blog post https://bit.ly/2xlZKGx

api_license_key_plugin.php

<?php

  require_once 'vendor/autoload.php';

  $http_client = new \GuzzleHttp\Client();
  $base_url    = 'https://api.example.com/';

  $error_message   = '';
  $success_message = '';
  $logged_in       = false;

  if (!empty($_POST) 
      && $_POST['user'] 
        && $_POST['password']) {

    $action = esc_html($_GET['action']);

    if ('auth' === $action) {
      $request_url = $base_url . '/license';
    } else if ('activate' === $action) {
      $request_url = $base_url . '/license/activate';
    } else if ('deactivate' === $action) {
      $request_url = $base_url . '/license/deactivate';
    }

    try {

      $site_url = parse_url($_SERVER['HTTP_HOST']);

      $response = $http_client->request('POST', 
        $request_url, [
          'auth'        => [esc_html($_POST['user']), esc_html($_POST['password'])],
          'form_params' => ['url' => esc_url($site_url['host'])]
        ]
      );

      if ($response->getStatusCode() === 200) {

        $json = json_decode($response->getBody());

        if ('auth' === $action) {

          $logged_in       = true;
          $license_key     = $json->license_key;
          $sites           = $json->sites;
          $success_message = 'Athentication was successful!';

          if (in_array(get_domain_only($site_url['host']), $sites)) {
            add_option('license_key_is_confirmed', 1);
          } else {
            delete_option('license_key_is_confirmed');
          }
        } else if ('activate' === $action) {

          if ($json->status === 'success') {
            $success_message = 'API license key confirmed!';
            add_option('license_key_is_confirmed', 1);
          }

        } else if ('deactivate' === $action) {

          if ($json->status === 'success') {
            $success_message = 'API license key confirmed!';
            delete_option('license_key_is_confirmed');
          }
        }
      }
    } catch (GuzzleHttp\Exception\ClientException $error) {

      $json = json_decode($error->getResponse()->getBody());
      
      if ($json->message) {
        $error_message = $json->message;
      } else {
        $error_message = $error->getMessage();
      }
    } catch (GuzzleHttp\Exception\RequestException $error) {
      
      $error_message = $error->getMessage();
    }
  }

  function get_domain_only(string $url) {

    $url = filter_var($url, FILTER_SANITIZE_URL);
    return preg_replace('/(http\:\/\/|https\:\/\/|www\.)/', '', $url);
  }