Can take payment from flutter app via webview
change with credentials bkash provide on BkashApi.php file
public $base_url = 'https://checkout.sandbox.bka.sh/v1.2.0-beta';
public $app_key = '5tunt4masn6pv2hnvte1sb5n3j';
public $app_secret = '1vggbqd4hqk9g96o9rrrp2jftvek578v7d2bnerim12a87dbrrka';
public $username = 'sandboxTestUser';
public $password = 'hWD@8vtzw0';