WP Global Cart - ECB Currency Converter; Retrieves Euro foreign exchan

<?php

    /**
     * WP Global Cart - ECB Currency Converter
     *
     * Retrieves Euro foreign exchange reference rates from the ECB
     * and converts amounts between currencies using EUR as the base.
     */
    class WOOGC_Currency_Converter {

        /**
         * ECB daily XML feed.
         */
        const ECB_FEED_URL = 'https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml';

        /**
         * WordPress option used to store the rates.
         */
        const OPTION_RATES = 'woogc_ecb_currency_rates';

        /**
         * WordPress option used to store the date of the last
         * successfully downloaded ECB rates.
         */
        const OPTION_DATE = 'woogc_ecb_currency_rates_date';

        /**
         * WordPress option used to store the last update timestamp.
         */
        const OPTION_UPDATED = 'woogc_ecb_currency_rates_updated';

        /**
         * Convert an amount from one currency to another.
         *
         * Example:
         * 100 EUR -> USD
         * 100 RON -> USD
         *
         * @param float  $amount
         * @param string $from_currency
         * @param string $to_currency
         *
         * @return float|false
         */
        public static function convert( $amount, $from_currency, $to_currency ) {

            $amount = (float) $amount;

            $from_currency = strtoupper( trim( $from_currency ) );
            $to_currency   = strtoupper( trim( $to_currency ) );

            // Nothing to convert.
            if ( $from_currency === $to_currency ) {
                return $amount;
            }

            // Make sure we have current rates, or the latest available rates.
            $rates = self::get_rates();

            if ( ! is_array( $rates ) || empty( $rates ) ) {
                return false;
            }

            /*
             * EUR is the base currency.
             *
             * Example:
             * EUR = 1
             * USD = 1.1551
             * RON = 5.2568
             */
            $from_rate = 1.0;
            $to_rate   = 1.0;

            /*
             * Currency other than EUR must exist in the ECB rates.
             */
            if ( 'EUR' !== $from_currency ) {

                if ( ! isset( $rates[ $from_currency ] ) ) {
                    return false;
                }

                $from_rate = (float) $rates[ $from_currency ];
            }

            if ( 'EUR' !== $to_currency ) {

                if ( ! isset( $rates[ $to_currency ] ) ) {
                    return false;
                }

                $to_rate = (float) $rates[ $to_currency ];
            }

            /*
             * Convert:
             *
             * source currency -> EUR -> target currency
             *
             * Example RON -> USD:
             *
             * 100 / 5.2568 * 1.1551
             */
            return ( $amount / $from_rate ) * $to_rate;
        }

        /**
         * Get the currently stored ECB rates.
         *
         * This will attempt to update the rates once per day.
         * If today's update is unavailable, the previously stored
         * successful rates are returned.
         *
         * @return array|false
         */
        public static function get_rates() {

            $stored_rates = get_option( self::OPTION_RATES, array() );
            $stored_date  = get_option( self::OPTION_DATE, '' );

            /*
             * If we already have rates downloaded today, use them.
             */
            $today = current_time( 'Y-m-d' );

            if (
                ! empty( $stored_rates ) &&
                $stored_date === $today
            ) {
                return $stored_rates;
            }

            /*
             * Try to download fresh ECB rates.
             */
            $new_rates = self::update_rates();

            /*
             * Successful update.
             */
            if ( is_array( $new_rates ) && ! empty( $new_rates ) ) {
                return $new_rates;
            }

            /*
             * ECB unavailable / no new rates today.
             *
             * Fall back to the last successful rates.
             */
            if ( ! empty( $stored_rates ) ) {
                return $stored_rates;
            }

            return false;
        }

        /**
         * Download and store the latest ECB rates.
         *
         * @return array|false
         */
        public static function update_rates() {

            $response = wp_remote_get(
                self::ECB_FEED_URL,
                array(
                    'timeout'     => 15,
                    'redirection' => 3,
                    'sslverify'   => true,
                    'headers'     => array(
                        'Accept' => 'application/xml,text/xml',
                    ),
                )
            );

            if ( is_wp_error( $response ) ) {
                return false;
            }

            $status_code = wp_remote_retrieve_response_code( $response );

            if ( 200 !== $status_code ) {
                return false;
            }

            $body = wp_remote_retrieve_body( $response );

            if ( empty( $body ) ) {
                return false;
            }

            /*
             * Prevent XML external entity processing.
             */
            libxml_use_internal_errors( true );

            $xml = simplexml_load_string(
                $body,
                'SimpleXMLElement',
                LIBXML_NONET | LIBXML_NOCDATA
            );

            if ( false === $xml ) {
                libxml_clear_errors();
                return false;
            }

            libxml_clear_errors();

            $rates = array(
                'EUR' => 1.0,
            );

            /*
             * ECB XML structure:
             *
             * <Cube currency="USD" rate="1.1551"/>
             */
            $currency_nodes = $xml->xpath(
                '//*[local-name()="Cube"][@currency and @rate]'
            );

            if ( empty( $currency_nodes ) ) {
                return false;
            }

            foreach ( $currency_nodes as $node ) {

                $currency = strtoupper( (string) $node['currency'] );
                $rate     = (float) $node['rate'];

                if ( empty( $currency ) || $rate <= 0 ) {
                    continue;
                }

                $rates[ $currency ] = $rate;
            }

            /*
             * Make sure we got a reasonable set of rates.
             */
            if ( count( $rates ) < 2 ) {
                return false;
            }

            /*
             * Save only after successfully parsing the complete feed.
             *
             * This is important: a failed/invalid request must never
             * overwrite the last known-good rates.
             */
            $today = current_time( 'Y-m-d' );

            update_option(
                self::OPTION_RATES,
                $rates,
                false
            );

            update_option(
                self::OPTION_DATE,
                $today,
                false
            );

            update_option(
                self::OPTION_UPDATED,
                time(),
                false
            );

            return $rates;
        }

        /**
         * Get information about the currently stored rates.
         *
         * Useful for debugging/admin UI.
         *
         * @return array
         */
        public static function get_status() {

            return array(
                'date'    => get_option( self::OPTION_DATE, '' ),
                'updated' => get_option( self::OPTION_UPDATED, 0 ),
                'rates'   => get_option( self::OPTION_RATES, array() ),
            );
        }
    }