Adding Caching to a Legacy App

Quick wins for legacy apps: cache expensive queries and computed fragments in APCu (single server) or Redis (shared), cache full pages for anonymous users at a reverse proxy (Varnish, Nginx fastcgi_cache, or a CDN), and add Cache-Control headers to static assets. Wrap caching in a small helper so keys and TTLs are consistent, and invalidate on writes.

<?php
function remember(string $key, int $ttl, callable $compute)
{
    $value = apcu_fetch($key, $hit);
    if ($hit) {
        return $value;
    }
    $value = $compute();
    apcu_store($key, $value, $ttl);
    return $value;
}

$categories = remember('categories:v3', 600, function () use ($pdo) {
    return $pdo->query('SELECT id, name FROM categories ORDER BY name')->fetchAll(PDO::FETCH_ASSOC);
});
// After admin edits categories: apcu_delete('categories:v3');

Common pitfalls

• Full-page caching pages that contain user-specific data.
• APCu on multiple servers with inconsistent invalidation.
• No TTLs, so stale data lives forever.

Key points

• APCu per server, Redis shared
• Page caching for anonymous traffic
• Invalidate on writes; always set TTLs

Further reading: PHP manual: APCu