<?php
  // Freemius PHP SDK - https://github.com/Freemius/freemius-php-sdk
  require_once './php-sdk-master/freemius/Freemius.php';

  define( 'FS__API_SCOPE', 'developer' );
  define( 'FS__API_DEV_ID', 1234 );
  define( 'FS__API_PUBLIC_KEY', 'pk_YOUR_PUBLIC_KEY' );
  define( 'FS__API_SECRET_KEY', 'sk_YOUR_SECRET_KEY' );

  // Init SDK.
  $api = new Freemius_Api(FS__API_SCOPE, FS__API_DEV_ID, FS__API_PUBLIC_KEY, FS__API_SECRET_KEY);

  $licenses = $api->Api("/plugins/{$plugin_id}/licenses.json?search=" . urlencode($validated_license_key));

  $is_valid_license_key = (1 === count($licenses->licenses));