Debugging and Logging Legacy Apps

Configure error logging properly (log_errors=On, error_log path, error_reporting=E_ALL, display_errors=Off in production), use Xdebug 3 step debugging locally (xdebug.mode=debug), and introduce Monolog for structured logs with request IDs. Error tracking services (Sentry, Bugsnag) group exceptions and show stack traces with context — often the fastest way to discover what a legacy app is actually doing wrong.

<?php
use Monolog\Handler\StreamHandler;
use Monolog\Logger;
use Monolog\Formatter\JsonFormatter;

$requestId = $_SERVER['HTTP_X_REQUEST_ID'] ?? bin2hex(random_bytes(8));
$handler = (new StreamHandler('php://stderr', Logger::INFO))->setFormatter(new JsonFormatter());
$log = new Logger('app', [$handler]);
$log->pushProcessor(function (array $record) use ($requestId) {
    $record['extra']['request_id'] = $requestId;
    return $record;
});

$log->warning('Payment retry', ['order_id' => $orderId, 'attempt' => 2]);

Common pitfalls

• Debugging production by enabling display_errors.
• Leaving Xdebug installed in production (big slowdown).
• Logging without context, so log lines can't be correlated.

Key points

• Log errors, never display them in production
• Xdebug for local step debugging
• Structured logs with request IDs; error tracking

Further reading: Xdebug: step debugging