Caching

By the end of this lesson you'll be able to cache expensive work with the cache-aside pattern, pick the right store (file, APCu, or Redis), set TTLs and invalidate safely, and avoid the cache stampede that brings sites down — turning a 200ms page into a sub-20ms one.

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Part of the free Php course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Why Cache, and the Cache-Aside Pattern

A cache is a fast, temporary store for the result of slow work — a database query, an API call, a heavy calculation — so you only pay for that work once. Reading from a database might take milliseconds; reading from a cache takes microseconds, often 100x faster . Two words you'll use constantly: a hit means the value was in the cache (fast), and a miss means it wasn't, so you had to do the slow work.

The standard recipe is cache-aside (also called lazy loading): check the cache first; on a miss, run the query, store the result, then return it. It's popular because it only caches what's actually asked for, and if the cache is empty or down a miss simply falls through to the database. Watch the loop — four requests, but only two database calls.

That's the entire idea. Every cache below — file, APCu, Redis — is the same three steps ( check → miss → store ); only the place the data is kept changes.

2️⃣ A File-Based Cache (and TTL)

The simplest real cache writes each value to a file on disk — no extra software needed. The new idea here is the TTL ( time-to-live ): how many seconds a value is allowed to live before it's treated as expired and recomputed. Without a TTL a value can become stale — users keep seeing yesterday's data forever. Here the file's last-modified time tells us how old the entry is.

File caches are great for rendered HTML fragments or config that rarely changes. They're a little slower than memory (disk access) and don't share well across many servers — which is exactly what the next two stores fix.

3️⃣ APCu — In-Memory Caching

APCu is a cache built into PHP that keeps data in the server's RAM , so reads are measured in microseconds. You enable the apcu extension, then use apcu_store($key, $value, $ttl) and apcu_fetch($key, $found) . The catch: it's per-server (each web server has its own copy) and is wiped on restart . Perfect for a single machine; not for a cluster.

4️⃣ Redis & Memcached — A Shared Cache

Once you have more than one web server, a per-server cache isn't enough — they'd each cache different things. Redis and Memcached run as their own separate service that every server connects to over the network, so the cache is shared . Memcached is a lean key/value store; Redis adds richer types (lists, sets, hashes), persistence to disk, and more — it's the usual default today. Note invalidation at the end: when the underlying data changes, you del() the key so the next read recomputes.

5️⃣ OPcache and the Cache Stampede

OPcache is a different kind of cache: instead of your data , it caches your compiled code . PHP normally reparses and recompiles every .php file on every request; OPcache keeps the compiled bytecode in memory so that work happens once — typically a 2–3x speed-up for free . You write no code; it's switched on in php.ini .

Finally, the failure mode to know about: a cache stampede (or "dog-piling"). A hot key expires, and in the same instant dozens of requests all miss — so they all run the same expensive query at once and crush the database. The fixes: a short lock so only the first request recomputes while the rest wait or serve the old value; early expiration , where one request refreshes just before the TTL; and a little random jitter on TTLs so many keys don't expire on the very same second.

6️⃣ Your Turn

Time to wire up the pattern yourself. The script below is almost complete — fill each ___ using the 👉 hint, then run it and check it against the Output panel.

One more — this time about freshness . Compare an entry's age against the TTL so stale data is detected.

📋 Quick Reference — Caching in PHP

No code is filled in this time — just a brief and an outline. Write it yourself, run it on onecompiler.com/php or your own machine, then check your result against the expected output in the comments. This is the same write-run-check loop you'll use on every real cache.

Practice quiz

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