Fiber-Based Task Scheduler

Also Known As

TL;DR

Explanation

PHP Fibers (8.1) are stackful coroutines — a Fiber can suspend itself with Fiber::suspend(), returning control to the caller, and be resumed later with $fiber->resume(). A scheduler maintains a queue of Fibers and runs each until it suspends or completes. Combined with non-blocking I/O (event loops like ReactPHP/Revolt), this enables genuine concurrent I/O in a single PHP thread. This is the foundation of async PHP frameworks and how libraries like Amp v3 implement their concurrency model.

Common Misconception

Why It Matters

Common Mistakes

• Blocking I/O inside a Fiber — blocks the entire scheduler, defeating concurrency.
• Not calling Fiber::suspend() when waiting for I/O — the Fiber monopolises the scheduler.
• Creating Fibers for CPU-bound work — they share the same thread; use separate processes instead.
• Not handling Fiber exceptions — uncaught exceptions in a Fiber terminate it silently.

Code Examples

// Blocking I/O inside Fiber — no concurrency gained:
$fiber = new Fiber(function(): void {
    $data = file_get_contents('https://api.example.com/data'); // BLOCKS!
    Fiber::suspend($data); // Too late — already waited synchronously
});

// Cooperative scheduler with non-blocking I/O:
$scheduler = new SplQueue();

$addTask = function(Fiber $fiber) use ($scheduler) {
    $scheduler->enqueue($fiber);
};

// Run scheduler:
while (!$scheduler->isEmpty()) {
    $fiber = $scheduler->dequeue();
    if (!$fiber->isStarted()) $fiber->start();
    elseif ($fiber->isSuspended()) $fiber->resume();
    if ($fiber->isSuspended()) $scheduler->enqueue($fiber); // Re-queue
}

// Task: suspend at I/O point, resume when ready:
$task = new Fiber(function() use ($loop): void {
    $promise = $loop->httpGet('https://api.example.com');
    $data = Fiber::suspend($promise); // Suspend — scheduler runs other tasks
    echo $data;
});

References

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

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