paypal-ipn-currency-conversion.php

<?php
/**
 * When the PayPal IPN is called, we'll replace the WC_Order class with a subclass that performs a currency conversion.
 *
 * NB: untested.
 *
 * @author BrianHenryIE
 */

/**
 * @see WC_Gateway_Paypal_IPN_Handler::get_paypal_order()
 *
 * @hooked valid-paypal-standard-ipn-request
 *
 * @param array $posted `wp_unslash( $_POST )`
 */
function use_currency_conversion_order_subclass_for_paypal_ipn( $posted ): void {

    if ( ! class_exists(My_PayPal_Currency_Conversion_WC_Order::class ) ) {
        class My_PayPal_Currency_Conversion_WC_Order extends WC_Order
        {
            public function get_total( $context = 'view' )
            {
                $local_currency_total = parent::get_total( $context );

                $exchange_rate = 1.23;

                $euro_total = $local_currency_total * $exchange_rate;

                return $euro_total;
            }
        }
    }

    /**
     * When `wc_get_order()` is called, substitute the class.
     *
     * @see WC_Order_Factory::get_order();
     * @see wc_get_order()
     *
     * @param string|false $classname
     * @param bool|array $order_type Details about the order type.
     * @param int|bool $order_id
     */
    $substitute_class = function ( $classname, $order_type, $order_id ) {

        static $returned = false;

        if( ! $returned ) {
            $returned = true;
            return My_PayPal_Currency_Conversion_WC_Order::class;
        }

        return WC_Order::class;
    };
    add_filter( 'woocommerce_order_class', $substitute_class, 10, 3);

    // If the static flag above doesn't work, this should be run as soon as possible after the change is no longer needed:
    // remove_filter( 'woocommerce_order_class', $substitute_class );
}

/**
 * Hook in before `::valid_response()` which calls `::get_paypal_order()` which uses `wc_get_order()`.
 *
 * @see WC_Gateway_Paypal_IPN_Handler
 * @see WC_Gateway_Paypal_IPN_Handler::check_response()
 * @see WC_Gateway_Paypal_IPN_Handler::valid_response()
 * @see WC_Gateway_Paypal_IPN_Handler::get_paypal_order()
 */
add_action( 'valid-paypal-standard-ipn-request', 'use_currency_conversion_order_subclass_for_paypal_ipn', 1 );