add_action( 'send_headers', 'tgm_io_strict_transport_security' );
/**
 * Enables the HTTP Strict Transport Security (HSTS) header.
 *
 * https://thomasgriffin.io/enable-http-strict-transport-security-hsts-wordpress/
 *
 * @since 1.0.0
 */
function tgm_io_strict_transport_security() {

    header( 'Strict-Transport-Security: max-age=10886400' );

}