* @since 5.9.0 Stop using deprecated `FILTER_SANITIZE_STRING`.
	 *
	 * @return null|true|WP_Error Returns `null` when the form hasn't been submitted, there's a nonce error, or there's no logged in user.
	 *                            Returns `true` on success and an error object when an error is encountered redeeming the voucher.
	 */
	public function redeem_voucher() {

		if ( ! isset( $_REQUEST['lifterlms_voucher_nonce'] ) || ! wp_verify_nonce( sanitize_text_field( wp_unslash( $_REQUEST['lifterlms_voucher_nonce'] ) ), 'lifterlms_voucher_check' ) || ! get_current_user_id() ) {
			return null;
		}

		$voucher  = new LLMS_Voucher();
		$redeemed = $voucher->use_voucher( llms_filter_input_sanitize_string( INPUT_POST, 'llms_voucher_code' ), get_current_user_id() );

		if ( is_wp_error( $redeemed ) ) {
			llms_add_notice( $redeemed->get_error_message(), 'error' );
			return $redeemed;
		}