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Parallel ⋅ Processes and Threads

The Process and Parallel classes simplify writing and running PHP in parallel. A script written to be run in parallel must return a callable that will be run in a child process (or a thread if ext-parallel is installed). The callable receives a single argument – an instance of Channel that can be used to send data between the parent and child processes. Any serializable data can be sent across this channel. The Context object, which extends the Channel interface, is the other end of the communication channel.

In the example below, a child process or thread is used to call a blocking function (file_get_contents() is only an example of a blocking function, use http-client for non-blocking HTTP requests). The result of that function is then sent back to the parent using the Channel object. The return value of the child process callable is available using the Context::join() method.

Child process or thread

# child.php

use Amp\Parallel\Sync\Channel;

return function (Channel $channel): \Generator {
    $url = yield $channel->receive();

    $data = file_get_contents($url); // Example blocking function

    yield $channel->send($data);

    return 'Any serializable data';

Parent Process

# parent.php

use Amp\Loop;
use Amp\Parallel\Context;

Loop::run(function () {
	// Creates a context using Process, or if ext-parallel is installed, Parallel.
    $context = Context\create(__DIR__ . '/child.php');

    $pid = yield $context->start();

    $url = '';

    yield $context->send($url);

    $requestData = yield $context->receive();

    printf("Received %d bytes from %s\n", \strlen($requestData), $url);

    $returnValue = yield $context->join();

    printf("Child processes exited with '%s'\n", $returnValue);

Child processes are also great for CPU-intensive operations such as image manipulation or for running daemons that perform periodic tasks based on input from the parent.