PHP Fibers: Concurrency Without Threads in PHP 8.1

PHP Fibers — Cooperative Concurrency

PHP Fibers (introduced in PHP 8.1) bring cooperative multitasking to PHP. Unlike threads, Fibers are not truly parallel — they run cooperatively, switching control explicitly. They are the foundation for async PHP frameworks like ReactPHP and Amp v3.

<?php
$fiber = new Fiber(function(): string {
    echo "Fiber started\n";
    $value = Fiber::suspend('hello');  // pause, send 'hello' to caller
    echo "Fiber resumed with: $value\n";
    return 'fiber done';
});

$result1 = $fiber->start();    // starts fiber, runs until first suspend
echo "Main got: $result1\n";   // 'hello'
$result2 = $fiber->resume('world'); // resumes fiber with 'world'
echo "Fiber returned: " . $fiber->getReturn() . "\n";

Multiple Fibers — Simulated Concurrency

<?php
$fibers = [];

// Create 3 fibers that simulate async work
for ($i = 1; $i <= 3; $i++) {
    $fibers[] = new Fiber(function() use ($i): void {
        echo "Task $i: starting\n";
        Fiber::suspend();  // yield control
        echo "Task $i: step 2\n";
        Fiber::suspend();  // yield again
        echo "Task $i: done\n";
    });
}

// Start all fibers
foreach ($fibers as $f) $f->start();

// Run until all complete (simple event loop)
do {
    $running = false;
    foreach ($fibers as $f) {
        if ($f->isSuspended()) {
            $f->resume();
            $running = true;
        }
    }
} while ($running);

// Output: Task 1 starting, Task 2 starting, Task 3 starting,
//         Task 1 step 2, Task 2 step 2... (interleaved!)

Fiber vs Generator

<?php
// Generator: one direction — only yields OUT
// Fiber: bidirectional — can suspend from ANYWHERE in the call stack

function deepFunction(): void {
    // Can suspend from nested calls — impossible with generators!
    Fiber::suspend('deep suspend');
}

$f = new Fiber(function(): void {
    echo "Before\n";
    deepFunction();  // suspends from inside nested function!
    echo "After\n";
});

$val = $f->start();    // 'deep suspend'
$f->resume();          // continues after deepFunction()