La clase parallel\Runtime

basic-multithreading-test.php
<?php
use parallel\Runtime;
use parallel\Channel;
$test = "this var is not accesible in a thread";
// this function will be the threads
$thread_function = function (int $id, Channel $ch) {
 // delay the first thread to simulate better multithreading
 // second thread always finishes first
 $sleep = ($id == 2) ? 1 : 2;
 sleep($sleep);
 // print thread id
 // so it's clear second thread goes first
 // and also you can make sure multithreading is working
 var_dump("thread $id sleep $sleep");
 // try to capture globals, but it's not possible
 echo '$GLOBALS["test"] = ';
 @var_dump($GLOBALS["test"]);
 // the only way to share data is between channels
 $ch->send($sleep);
};
try {
 // each runtime represents a thread
 $r1 = new Runtime();
 $r2 = new Runtime();
 // channel where the date will be sharead
 $ch1 = new Channel();
 // args that will be sent to $thread_function
 $args = array();
 $args[0] = null;
 $args[1] = $ch1;
 // running thread 1
 $args[0] = 1;
 $r1->run($thread_function, $args);
 // running thread 2
 $args[0] = 2;
 $r2->run($thread_function, $args);
 // receive data from channel
 $x = $ch1->recv();
 $y = $ch1->recv();
 // close channel
 $ch1->close();
 echo "\nData received by the channel: $x and $y";
} catch (Error $err) {
 echo "\nError:", $err->getMessage();
} catch (Exception $e) {
 echo "\nException:", $e->getMessage();
}

<?php
// make sure that is really parallel execution (showcase2).
// Then you use echo within task-function there is a real chance that the application will hang.
// We should control Futures to avoid this behaviour.
// You will get different output on each execution.
use parallel\{Future, Runtime};
const THREADS_COUNT = 10;
const THREADS_I_MAX = 100;
// executed within thread
$task = static function (int $i, int $to): void {
 echo "[enter$i]";
 for ($j = 0; $j < $to; $j++) {
 echo $i;
 }
 echo "[exit$i]";
};
// creating a few threads
$runtimeList = [];
for ($i = 0; $i < THREADS_COUNT; $i++) {
 $runtimeList[] = new Runtime();
}
// run all threads
$futureList = [];
foreach ($runtimeList as $i => $runtime) {
 echo "[run$i]";
 $futureList[] = $runtime->run($task, [$i, THREADS_I_MAX]);
}
// waiting until all threads are done
// if you delete code bellow then your script will hang
do {
 usleep(1);
 $allDone = array_reduce(
 $futureList,
 function (bool $c, Future $future): bool {
 return $c && $future->done();
 },
 true
 );
} while (false === $allDone);
echo "done\n";

new Runtime() will reset some basic (before number 10+-) Linux signal. 
If you already set handle of signal by pcntl_signal function - you need do it again

<?php
// make sure that is really parallel execution
// you will get different output on each script run
use parallel\{Channel, Runtime};
const THREADS_COUNT = 5;
const THREADS_I_MAX = 10;
$ch = new Channel();
// executed within thread
$task = static function (Channel $ch, int $i, int $to): void {
 echo "[enter: $i]\n";
 for ($j = 0; $j < $to; $j++) {
 $ch->send($i);
 }
 echo "[exit: $i]\n";
};
// creating a few threads
$runtimeList = [];
for ($i = 0; $i < THREADS_COUNT; $i++) {
 $runtimeList[] = new Runtime();
}
// run all threads
foreach ($runtimeList as $i => $runtime) {
 echo "[run: $i]\n";
 $runtime->run($task, [$ch, $i, THREADS_I_MAX]);
}
// reading messages from channel
$queue = '';
for ($i = 0; $i < THREADS_COUNT * THREADS_I_MAX; $i++) {
 $queue .= $ch->recv();
}
$ch->close();
echo $queue . PHP_EOL;