The Complete Guide to Website Caching (Explained 2025)
The Complete Guide to Website Caching (Explained 2025)
Published on June 25, 2025 Last Updated on July 4, 2025
Written by
Morgan Frank - Specialist in Page Speed
Caching is one of the most effective ways to dramatically improve website performance. It’s the process of storing frequently accessed data in a temporary storage location (a “cache”) so that it can be retrieved much faster on subsequent requests. Instead of having to generate the same content from scratch every time, the server can simply serve the cached copy.
Think of it like this: Imagine you’re a chef who has to make the same dish (a webpage) over and over again. Without caching, you’d have to gather all the ingredients, chop them up, and cook them from scratch every single time someone orders the dish. With caching, you can prepare some of the ingredients (or even the entire dish) ahead of time and store them in the refrigerator. When someone orders the dish, you can quickly grab the pre-prepared ingredients (or the finished dish) and serve it much faster.
This guide focuses on server-side caching, which means the caching happens on your web server or a dedicated caching server. We’ll cover three main types: server-side caching (often called page caching), object caching, and opcode caching.
Before we dive into the different types of caching, here are the key takeaways:
Key Takeaways
Caching Reduces Server Load: By serving cached content, you reduce the amount of work your server has to do, allowing it to handle more traffic.
Caching Improves Page Speed: Retrieving data from a cache is much faster than generating it from scratch.
Different Types of Caching: Server-side caching, object caching, and opcode caching target different aspects of the content generation process.
Server-Side (Page) Caching: Caches the entire HTML output of a page. Best for pages that don’t change frequently.
Object Caching: Caches individual objects (like database query results, API responses, or rendered HTML fragments) in memory. Best for dynamic websites with frequently changing content.
Opcode Caching: Caches the compiled bytecode of PHP scripts, speeding up PHP execution.
Use a Combination: The most effective caching strategy often involves using a combination of these techniques.
Invalidation: When content is updated, it invalidate the cache.
1. Server-Side Caching (Page Caching)
1. What it is: Server-side caching, often called “page caching,” stores the entire HTML output of a webpage in a cache. When a user requests the page, the server checks if a cached version exists. If it does, the server serves the cached HTML directly, without having to execute any PHP code or query the database. This is the fastest type of caching.
2. How it works:
A user requests a page.
The web server checks if a cached version of the page exists.
If a cached version exists: The server serves the cached HTML directly to the user’s browser.
If a cached version does not exist: The server processes the request (executes PHP code, queries the database, etc.), generates the HTML output, serves it to the user, and stores a copy in the cache for future requests.
2. Best for: Pages that don’t change very frequently, like blog posts, static pages About Us, Contact Us), and product pages that are not updated often.
3. Not suitable for: Pages with highly personalized content (e.g., a user’s shopping cart) or content that changes very frequently (e.g., a real-time stock ticker).
4. Implementation:
Web Server Modules:
Apache: mod_cache (as discussed in the “Server Configuration” section).
Nginx: Nginx’s built-in caching features (using proxy_cache).
Caching Plugins (for CMSs like WordPress):
RapidLoad AI: A premium caching plugin.
W3 Total Cache: A popular free caching plugin.
WP Super Cache: Another popular free caching plugin.
LiteSpeed Cache:
CDNs: Content Delivery Networks (CDNs) also perform server-side caching.
2. Object Caching
What it is: Object caching stores the results of individual database queries, API responses, or other data objects in memory. Instead of querying the database or making an API call every time the data is needed, the application can retrieve it from the cache.
How it works:
The application needs a piece of data (e.g., the results of a database query).
It first checks if the data is already stored in the object cache.
If the data is in the cache (“cache hit”): The application retrieves the data from the cache.
If the data is not in the cache (“cache miss”): The application retrieves the data from the database (or other source), stores it in the object cache, and then uses it.
Best for: Dynamic websites where the same data is frequently accessed. For example, a blog might cache the list of recent posts, or an e-commerce site might cache product details.
Not suitable for: Data that changes extremely frequently, or data that is unique to each user (in that case, you’d need a separate cache entry for each user, which can consume a lot of memory).
Implementation:
Memcached: A popular, high-performance, distributed memory object caching system.
Redis: Another popular in-memory data store that can be used for object caching (and many other purposes). Redis is often preferred over Memcached for its more advanced features.
APCu: A PHP extension that provides an in-memory cache for PHP applications.
WordPress Plugins: Many WordPress caching plugins (like W3 Total Cache and WP Rocket) also include object caching functionality.
3. Opcode Caching (PHP Only)
What it is: PHP is an interpreted language, which means that the PHP code is converted into machine readable instructions (“opcodes”) every time it’s executed. This compilation process takes time. Opcode caching stores the compiled bytecode (the opcodes) in memory, so the PHP code doesn’t have to be recompiled every time it’s executed.
How it works:
A PHP script is executed for the first time.
The PHP interpreter compiles the script into bytecode.
The opcode cache stores the bytecode in memory.
On subsequent requests, the PHP interpreter checks the opcode cache. If the bytecode is found, it’s executed directly, skipping the compilation step.
Best for: All PHP-based websites. Opcode caching is a fundamental optimization for PHP performance.
Implementation:
OPcache: The most common opcode cache for PHP. It’s a PHP extension that’s often included and enabled by default in modern PHP installations.
The most effective caching strategy often involves using a combination of these techniques:
Opcode caching (OPcache): Always enabled for PHP websites.
Object caching (Memcached or Redis): For dynamic websites with frequently accessed data.
Server-side/Page caching: For pages that don’t change very frequently.
Browser caching: (As discussed in a previous section) – Instructing the user’s browser to store static assets locally.
CDN caching: (As discussed in a previous section) – Caching content on a globally distributed network of servers.
Conclusion
By strategically implementing these caching layers, you can dramatically reduce server load, improve age speed, and provide a much better user experience. Remember to always test your caching configuration thoroughly and monitor your website’s performance to ensure that your caching strategy is effective.
Determined to change that, he built RapidLoad — a smart, AI-driven tool that empowers site owners to dramatically improve speed scores, enhance user experience, and meet Google’s Core Web Vitals without needing to touch a single line of code.
