Understanding PHP 8’s JIT (Just-in-Time) Compilation and How It Impacts Performance

When PHP 8 was released, one of the most talked-about features was JIT compilation, or Just-in-Time compilation. For years, PHP has relied on its Zend Engine to interpret code line by line at runtime. But with JIT in the picture, there’s a new layer of performance optimization that developers can tap into.

If you're still wondering what JIT actually does and whether it will turbocharge your app — this article breaks it down simply and practically.

What Is JIT Compilation?

Traditionally, PHP code goes through a few steps:

1. It’s parsed into opcodes by the Zend Engine.

2. The opcodes are interpreted and executed at runtime.

With JIT, PHP compiles these opcodes into machine code on the fly (hence the name just-in-time), which your CPU can execute directly — no need for interpretation.

This technique is borrowed from other languages like Java and .NET, where JIT has already proven its ability to significantly improve performance in certain scenarios.

What Is JIT Compilation?

Traditionally, PHP code goes through a few steps:

1. It’s parsed into opcodes by the Zend Engine.

2. The opcodes are interpreted and executed at runtime.

With JIT, PHP compiles these opcodes into machine code on the fly (hence the name just-in-time), which your CPU can execute directly — no need for interpretation.

This technique is borrowed from other languages like Java and .NET, where JIT has already proven its ability to significantly improve performance in certain scenarios.

How Does It Work Under the Hood?

When JIT is enabled:

• PHP uses a monitoring system that identifies which parts of your code are “hot” (i.e., frequently used).

• These hot paths are compiled into native machine code.

• The next time those parts are executed, they’re run much faster.

This doesn’t mean every part of your code gets compiled — only the parts PHP determines are worth the effort.

Real-World Performance Gains

Now, let’s address the real question: Does JIT make PHP apps faster?

The answer is… it depends.

Where JIT Shines:

• CPU-intensive tasks like image processing, mathematical computations, or custom algorithms.

• Long-running scripts (e.g., CLI scripts or daemons).

• Projects using PHP for AI, simulations, or data-heavy operations.

In benchmarks, some computational tasks saw up to 2x or more performance boosts.

Where JIT Has Little Impact:

• Typical web applications (Laravel, WordPress, etc.)

• IO-bound tasks (like database calls, file operations)

For most web apps, the bottleneck is usually the database or network latency — not the execution speed of PHP code itself. In such cases, JIT won’t move the needle much.

Enabling JIT in PHP 8

If you want to experiment, enabling JIT is straightforward:

1. Edit your php.ini file:

   opcache.enable=1
   opcache.jit_buffer_size=100M
   opcache.jit=tracing

2. Restart your server (Apache/Nginx/PHP-FPM).

3. Verify JIT is working:

   php -i | grep JIT

   You should see something like: JIT => On.

   Should You Use JIT in Production?

   If your application does heavy computation, it’s definitely worth testing with JIT. You might see substantial gains — especially on CLI or API-heavy apps.

   But for most Laravel/WordPress developers, JIT won’t magically make things faster. It’s more of a low-level engine upgrade that sets the stage for PHP to grow into other domains.

   Still, it’s an exciting sign of where the language is headed.

   Final Thoughts

   JIT compilation in PHP 8 isn’t a silver bullet — but it is an important advancement. It won’t magically speed up your Laravel app overnight, but it does unlock potential in areas PHP couldn’t previously compete.

   If you’re curious and like performance optimization, give it a try. Even just understanding how it works will make you a better PHP developer.

   Let me know what kind of results you get — the PHP community is still exploring what’s possible with this new superpower.