*     @type string agree_to_terms  (Required if `llms_are_terms_and_conditions_required()` are required) If terms & conditions are required this should be "yes" for agreement.
 *     @type string payment_gateway (Optional) ID of a registered LLMS_Payment_Gateway which will be used to process the order.
 *     @type string coupon_code     (Optional) Coupon code to be applied to the order.
 * }
 * @return array
 */
function llms_setup_pending_order( $data = array() ) {

	/**
	 * Filters the order data before setting up the pending order.
	 *
	 * @since Unknown.
	 *
	 * @param array $data Array of input data from a checkout form.
	 */
	$data = apply_filters( 'llms_before_setup_pending_order', $data );

	// Request keys that can be submitted with or without the `llms_` prefix.
	$keys = array(
		'llms_agree_to_terms',
		'llms_coupon_code',
		'llms_plan_id',
	);
	foreach ( $keys as $key ) {
		if ( isset( $data[ $key ] ) ) {
			$data[ str_replace( 'llms_', '', $key ) ] = $data[ $key ];
		}
	}

	$err = new WP_Error();

	// Check t & c if configured.
	if ( llms_are_terms_and_conditions_required() ) {
		if ( ! isset( $data['agree_to_terms'] ) || ! llms_parse_bool( $data['agree_to_terms'] ) ) {
			$err->add( 'terms-violation', sprintf( __( 'You must agree to the %s to continue.', 'lifterlms' ), get_the_title( get_option( 'lifterlms_terms_page_id' ) ) ) );
			return $err;
		}
	}

	// We must have a plan_id to proceed.
	if ( empty( $data['plan_id'] ) ) {
		$err->add( 'missing-plan-id', __( 'Missing an Access Plan ID.', 'lifterlms' ) );
		return $err;
	}

	// Validate the plan is a real plan.
	$plan = llms_get_post( absint( $data['plan_id'] ) );
	if ( ! $plan || 'llms_access_plan' !== $plan->get( 'type' ) ) {
		$err->add( 'invalid-plan-id', __( 'Invalid Access Plan ID.', 'lifterlms' ) );
		return $err;
	}

	// Used later.
	$coupon_id = null;
	$coupon    = false;

	// If a coupon is being used, validate it.
	if ( ! empty( $data['coupon_code'] ) ) {

		$data['coupon_code'] = sanitize_text_field( $data['coupon_code'] );

		$coupon_id = llms_find_coupon( $data['coupon_code'] );

		// Coupon couldn't be found.
		if ( ! $coupon_id ) {
			$err->add( 'coupon-not-found', sprintf( __( 'Coupon code "%s" not found.', 'lifterlms' ), $data['coupon_code'] ) );
			return $err;
		}

		// Coupon is real, make sure it's valid for the current plan.
		$coupon = llms_get_post( $coupon_id );
		$valid  = $coupon->is_valid( $data['plan_id'] );

		// If the coupon has a validation error, return an error message.
		if ( is_wp_error( $valid ) ) {
			$err->add( 'invalid-coupon', $valid->get_error_message() );
			return $err;
		}
	}

	// If payment is required, verify we have a gateway.
	if ( $plan->requires_payment( $coupon_id ) && empty( $data['payment_gateway'] ) ) {
		$err->add( 'missing-gateway-id', __( 'No payment method selected.', 'lifterlms' ) );
		return $err;
	}

	$gateway_id    = empty( $data['payment_gateway'] ) ? 'manual' : $data['payment_gateway'];
	$gateway_error = llms_can_gateway_be_used_for_plan( $gateway_id, $plan );
	if ( is_wp_error( $gateway_error ) ) {
		return $gateway_error;
	}

	if ( empty( $data['customer'] ) ) {
		$err->add( 'missing-customer', __( 'Missing customer information.', 'lifterlms' ) );
		return $err;
	}

	// Update the customer.
	if ( ! empty( $data['customer']['user_id'] ) ) {
		$person_id = llms_update_user( $data['customer'], 'checkout', compact( 'plan' ) );
	} else {
		$person_id = llms_register_user( $data['customer'], 'checkout', true, compact( 'plan' ) );
	}

	// Validation or registration issues.
	if ( is_wp_error( $person_id ) ) {
		return $person_id;
	}

	// This will likely never actually happen unless there's something very strange afoot.
	if ( ! is_numeric( $person_id ) ) {

		$err->add( 'account-creation', __( 'An unknown error occurred when attempting to create an account, please try again.', 'lifterlms' ) );
		return $err;

	}

	// Ensure the new user isn't enrolled in the product being purchased.
	if ( llms_is_user_enrolled( $person_id, $plan->get( 'product_id' ) ) ) {

		$product = $plan->get_product();
		$err->add(
			'already-enrolled',
			sprintf(
				// Translators: %2$s = The product type (course/membership); %1$s = product permalink.
				__( 'You already have access to this %2$s! Visit your dashboard <a href="%1$s">here.</a>', 'lifterlms' ),
				llms_get_page_url( 'myaccount' ),
				$product->get_post_type_label()
			)
		);
		// Prevent double displaying a notice to already enrolled students in the product being purchased.
		add_filter( 'llms_display_checkout_form_enrolled_students_notice', '__return_false' );

		return $err;
	}

	$person  = llms_get_student( $person_id );
	$gateway = llms()->payment_gateways()->get_gateway_by_id( $gateway_id );

	/**
	 * Filter the return of pending order setup data.
	 *
	 * @since 3.30.1
	 *
	 * @param array $setup {
	 *     Data used to create the pending order.
	 *
	 *     @type LLMS_Student $person Student object.
	 *     @type LLMS_Access_Plan $plan Access plan object.
	 *     @type LLMS_Payment_Gateway $gateway Instance of the selected gateway.
	 *     @type LLMS_Coupon|false $coupon Coupon object or false if none used.