WooCommerce Admin filter Orders by currency

<?php

/**
 * Plugin Name: Filter Orders by currency
 * Description: Adds the ability to filter orders by currency.
 * Author: Gilles Goetsch
 * Author URI: https://gillesgoetsch.ch/
 * Version: 1.0.0
 * Text Domain: wc-filter-orders
 * Reference: http://www.skyverge.com/product/woocommerce-filter-orders/
 *
 * GitHub Plugin URI: bekarice/woocommerce-filter-orders/
 * GitHub Branch: master
 *
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 *
 * @package   WC-Filter-Orders-currency
 * @author    Gilles Goetsch
 * @category  Admin
 * @license   http://www.gnu.org/licenses/gpl-3.0.html GNU General Public License v3.0
 *
 */

// fire it up!
add_action( 'plugins_loaded', 'wc_filter_orders_by_currency' );

/**
 * Plugin Description
 *
 * Adds custom filtering to the orders screen to allow filtering by currency used.
 */
 class WC_Filter_Orders_By_currency {


    const VERSION = '1.0.0';

    /** @var WC_Filter_Orders_By_currency single instance of this plugin */
    protected static $instance;

    /**
     * WC_Filter_Orders constructor.
     *
     * @since 1.0.0
     */
    public function __construct() {

        // load translations
        add_action( 'init', array( $this, 'load_translation' ) );

        if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {

            // adds the currency filtering dropdown to the orders page
            add_action( 'restrict_manage_posts', array( $this, 'filter_orders_by_currency_used' ) );

            // makes currencys filterable
            add_action('pre_get_posts', array( $this, 'filter_query_by_currency'));
        }
    }


    /** Plugin methods ***************************************/


    /**
     * Adds the currency filtering dropdown to the orders list
     *
     * @since 1.0.0
     */
    public function filter_orders_by_currency_used() {
        global $typenow;

        if ( 'shop_order' === $typenow ) {

            $currencies = array('CHF','EUR','GBP','USD'); // could be automated

            if ( ! empty( $currencies ) ) : ?>

                <select name="_currency_used" id="dropdown_currency_used">
                    <option value="">
                        <?php esc_html_e( 'Currency', 'wc-filter-orders' ); ?>
                    </option>
                    <?php foreach ( $currencies as $currency ) : ?>
                        <option value="<?php echo esc_attr( $currency ); ?>" <?php echo esc_attr( isset( $_GET['_currency_used'] ) ? selected( $currency, $_GET['_currency_used'], false ) : '' ); ?>>
                            <?php echo esc_html( $currency ); ?>
                        </option>
                    <?php endforeach; ?>
                </select>
            <?php endif;
        }
    }


    function filter_query_by_currency($query) {
        global $typenow;

        if ( 'shop_order' === $typenow && isset( $_GET['_currency_used'] ) && ! empty( $_GET['_currency_used'] ) ) {
            $query->set('meta_query', array( array(
              'key' => '_order_currency',
              'value' => $_GET['_currency_used']
            ) ) );
        }
    }


    /** Helper methods ***************************************/


    /**
     * Load Translations
     *
     * @since 1.0.0
     */
    public function load_translation() {
        // localization
        load_plugin_textdomain( 'wc-filter-orders', false, dirname( plugin_basename( __FILE__ ) ) . '/i18n/languages' );
    }


    /**
     * Main WC_Filter_Orders_By_currency Instance, ensures only one instance
     * is/can be loaded
     *
     * @since 1.1.0
     *
     * @see wc_filter_orders_by_currency()
     * @return WC_Filter_Orders_By_currency
     */
    public static function instance() {
        if ( is_null( self::$instance ) ) {
            self::$instance = new self();
        }
        return self::$instance;
    }


}

/**
 * Returns the One True Instance of WC_Filter_Orders_By_currency
 *
 * @since 1.1.0
 *
 * @return \WC_Filter_Orders_By_currency
 */
function wc_filter_orders_by_currency() {
    return WC_Filter_Orders_By_currency::instance();
}

/*
if(is_admin()) {
    wc_filter_orders_by_currency(); // do this if added to the functions.php
}
*/