parallel/docs/processes.md

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Responses	/processes

Process simplifies 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. 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 Process object is the other end of the communication channel, as it implements Context, which extends Channel.

In the example below, a child process is used to call a blocking function (file_get_contents() is only an example of a blocking function, use Artax 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 Process::join() method.

Child Process

# 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\Process;

Loop::run(function () {
    $process = new Process(__DIR__ . '/child.php');

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

    $url = 'https://google.com';

    yield $process->send($url);

    $requestData = $process->receive();

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

    $returnValue = $process->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.