Legacy Refactor with Comparative Analysis

parallel_testing.php

<?php

function processOrder(Order $order): Invoice
{
    $invoiceA = $order->processOrderOnLegacyLogic();
    $invoiceB = $order->processOrderOnRefactoredLogic();

    if($invoiceA->getTotal() !== $invoiceB->getTotal()) {
        $this->log->error(
            'Invoice comparison failed.',
            [
                'order' => $order,
                'invoiceA' => $invoiceA,
                'invoiceB' => $invoiceB
            ]
        );
    }

    return $invoiceA;
}

function processOrderOnLegacyLogic(Order $order): Invoice
{
    // here be dragons
}

function processOrderOnRefactoredLogic()
{
    // all that is good
}