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Amit Merchant

Amit Merchant

A blog on PHP, JavaScript, and more

Fibers or Coroutines for asynchronous programming in PHP 8.1

There has been a scarcity of discussion in PHP land when it comes to asynchronous PHP. Maybe, PHP developers don’t seem to be interested in the very idea of it.

But there’s a selected set of people such as the library and framework authors who would benefit from asynchronous programming. And that’s exactly why libraries like ReactPHP exists.

In one of my articles, I have explained how you can use JavaScript-style promises in PHP using ReactPHP. That’s indeed interesting if you’re coming from languages such as JavaScript and want to achieve similar behavior in single-threaded languages like PHP.

As you can tell, this can only be achieved using third-party libraries up until now. But, it seems like things are going to change in PHP 8.1.

Fibers or Coroutines in PHP 8.1

An RFC targetted for PHP 8.1, currently in voting phase, is proposing to add Fibers (or Coroutines) in PHP natively.

The RFC would add a Fiber class and the corresponding reflection class ReflectionFiber in PHP 8.1.

And essentially,

Fibers allow the creation of full-stack, interruptible (suspendable) functions that can be used to implement cooperative multitasking in PHP. These are also known as coroutines or green-threads.

Creating a Fiber

Fiber objects can be created by passing a callable to the constructor of the Fiber class like so.

new Fiber(callable $callback)

After this, you can call a set of predefined methods (start(), suspend(), and resume()) on this object to perform non-blocking I/O implementations.

Let’s check a simple example from the RFC below.

$fiber = new Fiber(function (): void {
    $value = Fiber::suspend('fiber');
    echo "Value used to resume fiber: ", $value, "\n";
$value = $fiber->start();
echo "Value from fiber suspending: ", $value, "\n";

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

From the above snippet, we can conclude that Fibers involves two operations:

  • Suspension of the program execution
  • Resumption of the program execution

Suspend the program execution

As you can tell, we have created a fiber that immediately suspends with the string “fiber” using the Fiber::suspend method. This string is returned from the call to $fiber->start() which will print “Value from fiber suspending: fiber”.

Resume the program execution

The fiber is then resumed with the string “test” using $fiber->resume('test'), which is returned from the call to Fiber::suspend(). This will print “Value used to resume fiber: test”.

Throw exception from a fiber

You can also throw an exception from a fiber using Fiber->throw(). So, a suspended fiber can be extended by throwing an exception from Fiber::suspend() as well.

In closing

So, this was your first look at Fibers/Coroutines in PHP 8.1. There are a lot of other things about Fibers that I have not discussed in this article such as how Fibers are different from Generators, the performance of fibers, support, and other things.

If you are interested, I would recommend checking out the official RFC.

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