Fibers — Cooperative Concurrency (PHP 8.1)

TL;DR

Explanation

Fiber::create($callback) creates a fiber. $fiber->start($args) begins execution. Fiber::suspend($value) pauses and returns value to caller. $fiber->resume($value) continues from the suspension point. Fibers are single-threaded — not parallel. They enable: event-loop integration (ReactPHP, Revolt, Swoole), cooperative multitasking, coroutine-style async code without callbacks. The Revolt event loop uses Fibers to implement async/await-like patterns. Unlike generators, Fibers can suspend from any call depth (generators only yield from the generator function itself). This is the PHP equivalent of Python's asyncio coroutines.

Common Misconception

Why It Matters

Common Mistakes

• Expecting Fibers to use multiple CPU cores — they're single-threaded.
• Using Fibers without an event loop — fibers alone don't help without something to schedule them.
• Confusing with PHP generators — fibers can suspend from any call depth.

Code Examples

// Generators only yield from generator function:
function gen() {
    yield doSomething(); // Only works at generator level
}

$fiber = new Fiber(function(): void {
    $value = Fiber::suspend('first');
    echo 'Resumed with: ' . $value . PHP_EOL;
});

$yielded = $fiber->start(); // 'first'
$fiber->resume('hello'); // 'Resumed with: hello'

// Real use: Revolt event loop
// use Revolt\EventLoop;
// EventLoop::run();

References

• ↗ https://www.php.net/manual/en/class.fiber.php

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