Fibers

Learn to write thread-like coroutines with the Fiber class.

What is a Fiber?

PHP 8.1 added the Fiber class: a coroutine, a thread-like unit of cooperative execution. It is a piece of code that can pause and resume within the same call stack.

Create one with new Fiber(fn). The ->start() method runs it; Fiber::suspend() pauses and returns to the caller; ->resume() continues.

Fibers are not OS threads; they enable cooperative multitasking. No other fiber runs unless the current one pauses.

Data between suspend and resume

Inside a fiber, Fiber::suspend($v) halts execution and returns $v to the start() call. The caller resumes with ->resume($w); $w becomes the result of suspend().

This builds a two-way value flow between the fiber and the caller. When the fiber completes, getReturn() reads its return value.

Typical uses: asynchronous I/O, producer-consumer patterns and event loop implementations.

Exercise A basic Fiber flow

Create a Fiber: when started it should print fiber-start, pause with Fiber::suspend(), then print fiber-end after resume.

Outside, call start(), print main1, call resume(), print main2. Output order should be fiber-start, main1, fiber-end, main2.

Show example solution

$f = new Fiber(function (): void {
  echo "fiber-start\n";
  Fiber::suspend();
  echo "fiber-end\n";
});

$f->start();
echo "main1\n";
$f->resume();
echo "main2\n";

Test yourself

Which method starts a Fiber?
- run()
- begin()
- start()
- launch()
Show answer

start() — Correct. The start() method of the Fiber class begins the fiber and sends the initial value.

Fiber suspend call

$f = new Fiber(function ($n) {
  echo $n;
  ____($n + 1);
});

Show answer

$f = new Fiber(function ($n) {
  echo $n;
  Fiber::suspend($n + 1);
});

Match the Fiber concepts

Fiber

coroutine class
start

begins the fiber
suspend

pauses the fiber
resume

continues a paused fiber

Next lesson: First-class callable syntax

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