PHP 8.0 JIT: What It Really Does (and Doesn’t) Do

🌟 Introduction

PHP 8.0 introduced Just-In-Time compilation (JIT) with high expectations. Developers imagined JavaScript-like performance boosts and dreams of CPU-bound PHP games. But what's the real story?

In this article, we’ll:

• Explain what JIT does

• Show how to activate it

• Run benchmarks with and without it

• Reveal where it truly shines (and where it doesn’t)

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🧠 What is JIT?

JIT stands for Just-In-Time Compilation. Unlike traditional interpreters, which parse and execute line-by-line, a JIT-enabled engine compiles parts of the code at runtime into native machine code, which can be executed directly by the CPU.

✨ JIT vs OPCache

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🔧 How to Enable JIT in PHP 8.0

Make sure you're using PHP 8.0 or later. Then edit your php.ini (often found via php --ini):

opcache.enable=1
opcache.jit_buffer_size=100M
opcache.jit=1255

📄 Meaning of opcache.jit=1255

This config tells PHP to use the tracing JIT with maximum optimization level (function + loop tracing). You can tweak it for different performance profiles:

⚠️ JIT has no effect unless opcache.jit_buffer_size is > 0.

To confirm it works:

php -i | grep JIT

Should show something like:

opcache.jit => 1255 => 1255
opcache.jit_buffer_size => 100M => 100M

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🔢 Benchmark Time

Let's look at 3 simple benchmarks.

1. Simple Loop (CPU-bound)

$start = microtime(true);

$sum = 0;
for ($i = 0; $i < 100000000; $i++) {
    $sum += $i;
}

echo "Time: ".(microtime(true) - $start)."s\n";

Results:

2. Web API Simulation

// simulate DB call delay// simulate DB call delay\usleep(100);
echo "Hello, world!";

Results:

No gain here—JIT doesn’t help with I/O.

3. Mandelbrot Fractal

// Source: official php-src/bench.php examples

Results:

This test is computationally expensive and benefits a lot from JIT.

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🫶 Should You Use It?

In short:

• Don’t expect miracles for web workloads.

• Do try it for CLI or intensive math tasks.

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📊 Final Thoughts

The JIT engine in PHP 8.0 is a great evolution, especially for non-traditional PHP use cases. It brings PHP closer to compiled language performance for math-heavy scenarios.

But for your typical Laravel or Symfony app? The difference is minimal. If you're running CPU-intensive background jobs, however, go ahead and experiment with JIT—you might be surprised.

📅 Fun fact: The real performance gain for web apps still comes from OPCache, code refactoring, and DB optimizations, not JIT.

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Written for OpenSource My Friend — your Laravel and PHP performance playground.