periodic_write.php

<?php

use React\EventLoop\Factory;
use React\Filesystem\Filesystem;
use function React\Promise\Stream\unwrapWritable;

require __DIR__ . '/../../vendor/autoload.php';

$loop = Factory::create();
$filesystem = Filesystem::create($loop);

$fileStream = unwrapWritable($filesystem->file('test.txt')->open('cw'));

$loop->addPeriodicTimer(1, function () use ($fileStream) {
    $fileStream->write("Hello world\n");
});

$loop->run();