Fibers overview

Fibers represent full-stack, interruptible functions. Fibers may be suspended from anywhere in the call-stack, pausing execution within the fiber until the fiber is resumed at a later time.

Fibers pause the entire execution stack, so the direct caller of the function does not need to change how it invokes the function.

Execution may be interrupted anywhere in the call stack using Fiber::suspend (that is, the call to Fiber::suspend may be in a deeply nested function or not even exist at all).

Unlike stack-less Generators, each Fiber has its own call stack, allowing them to be paused within deeply nested function calls. A function declaring an interruption point (that is, calling Fiber::suspend) need not change its return type, unlike a function using yield which must return a Generator instance.

Fibers can be suspended in any function call, including those called from within the PHP VM, such as functions provided to array_map or methods called by foreach on an Iterator object.

Once suspended, execution of the fiber may be resumed with any value using Fiber::resume or by throwing an exception into the fiber using Fiber::throw. The value is returned (or exception thrown) from Fiber::suspend.

Note: Prior to PHP 8.4.0, switching fibers during the execution of an object destructor was not allowed.

<?php
$fiber = new Fiber(function (): void {
   $value = Fiber::suspend('fiber');
   echo "Value used to resume fiber: ", $value, PHP_EOL;
});

$value = $fiber->start();

echo "Value from fiber suspending: ", $value, PHP_EOL;

$fiber->resume('test');
?>

Value from fiber suspending: fiber
Value used to resume fiber: test