public static function output( $atts ) {

		global $wp;

		$atts = $atts ? $atts : array();

		$atts['cols'] = isset( $atts['cols'] ) ? $atts['cols'] : 2;

		self::$uid = get_current_user_id();

		$atts['gateways']         = llms()->payment_gateways()->get_enabled_payment_gateways();
		$atts['selected_gateway'] = llms()->payment_gateways()->get_default_gateway();

		$atts['order_key'] = '';

		$atts['field_data'] = array();
		if ( isset( $_POST ) && isset( $_POST['action'] ) && 'create_pending_order' === $_POST['action'] ) { // phpcs:ignore WordPress.Security.NonceVerification.Missing
			$atts['field_data'] = wp_unslash( $_POST ); // phpcs:ignore WordPress.Security.NonceVerification.Missing
		} elseif ( self::$uid ) {
			$atts['field_data'] = get_current_user_id();
		}

		self::checkout_wrapper_start();

		/**
		 * Allows gateways or third parties to output custom errors before
		 * any core logic is executed.
		 *
		 * This filter returns `false` by default. To output custom errors return
		 * the error message as a string that will be displayed on screen.
		 *
		 * @since Unknown
		 *
		 * @param bool|string $pre_error A custom error message.
		 */
		$err = apply_filters( 'lifterlms_pre_checkout_error', false );
		if ( $err ) {
			self::error( $err );
			self::checkout_wrapper_end();
			return;
		}

		llms_print_notices();

		// purchase step 1.
		if ( isset( $_GET['plan'] ) && is_numeric( $_GET['plan'] ) ) {

			$plan_id = llms_filter_input( INPUT_GET, 'plan', FILTER_SANITIZE_NUMBER_INT );

			// Only retrieve if plan is a llms_access_plan and is published.
			if ( 0 === strcmp( get_post_status( $plan_id ), 'publish' ) && 0 === strcmp( get_post_type( $plan_id ), 'llms_access_plan' ) ) {

				$coupon = llms()->session->get( 'llms_coupon' );

				if ( isset( $coupon['coupon_id'] ) && isset( $coupon['plan_id'] ) ) {
					if ( $coupon['plan_id'] == $_GET['plan'] ) {
						$atts['coupon'] = new LLMS_Coupon( $coupon['coupon_id'] );
					} else {
						llms()->session->set( 'llms_coupon', false );
						$atts['coupon'] = false;
					}
				} else {
					$atts['coupon'] = false;
				}

				// Use posted order key to resume a pending order.
				if ( isset( $_POST['llms_order_key'] ) ) { // phpcs:ignore WordPress.Security.NonceVerification.Missing
					$atts['order_key'] = llms_filter_input_sanitize_string( INPUT_POST, 'llms_order_key' );

					// Attempt to locate a pending order.
				} elseif ( self::$uid ) {
					$pending_order = llms_locate_order_for_user_and_plan( self::$uid, $plan_id );
					if ( $pending_order ) {
						$order             = llms_get_post( $pending_order );
						$atts['order_key'] = ( 'llms-pending' === $order->get( 'status' ) ) ? $order->get( 'order_key' ) : '';
					}
				}

				$atts = self::setup_plan_and_form_atts( $plan_id, $atts );

				/**
				 * Filter the number of columns used to render the checkout/enrollment form.
				 *
				 * @since Unknown.
				 * @since 5.0.0 Added `$form_location` parameter.
				 *
				 * @param int $cols Number of columns. Accepts 1 or 2.
				 * @param LLMS_Access_Plan $plan Access plan object.
				 * @param string $form_location Form location ID.
				 */
				$atts['cols'] = apply_filters( 'llms_checkout_columns', ( $atts['is_free'] || ! $atts['form_fields'] ) ? 1 : $atts['cols'], $atts['plan'], $atts['form_location'] );

				self::checkout( $atts );

			} else {

				self::error( __( 'Invalid access plan.', 'lifterlms' ) );

			}
		} elseif ( isset( $wp->query_vars['confirm-payment'] ) ) {

			$order_key = llms_filter_input_sanitize_string( INPUT_GET, 'order' );
			$order     = $order_key ? llms_get_order_by_key( $order_key ) : false;
			if ( ! $order ) {
				self::error( __( 'Could not locate an order to confirm.', 'lifterlms' ) );
				self::checkout_wrapper_end();
				return;
			}

			$atts = self::setup_plan_and_form_atts( $order->get( 'plan_id' ), $atts );

			if ( $order->get( 'coupon_id' ) ) {
				$atts['coupon'] = new LLMS_Coupon( $order->get( 'coupon_id' ) );
			} else {
				$atts['coupon'] = false;
			}

			$atts['selected_gateway'] = llms()->payment_gateways()->get_gateway_by_id( $order->get( 'payment_gateway' ) );

			self::confirm_payment( $atts );

		} else {

			self::error( sprintf( __( 'Your cart is currently empty. Click <a href="%s">here</a> to get started.', 'lifterlms' ), llms_get_page_url( 'courses' ) ) );

		}

		self::checkout_wrapper_end();

	}