Here's a more substantial example of how to use the run functional API.

<?php
/*********************************************
 * Sample parallel functional API
 * 
 * Scenario
 * -------------------------------------------
 * Given a large number of rows of
 * data to process, divide the work amongst
 * a set of workers.  Each worker is responsible
 * for finishing their assigned task.
 * 
 * In the code below, assume we have arbitrary 
 * start and end IDs (rows) - we will try to
 * divide the number of IDs (rows) evenly
 * across 8 workers.  The workers will get the
 * following batches to process to completion:
 *
 * Total number of IDs (rows): 1371129
 * Each worker will get 171392 IDs to process
 *
 * Worker 1: IDs from 11001 to 182393
 * Worker 2: IDs from 182393 to 353785
 * Worker 3: IDs from 353785 to 525177
 * Worker 4: IDs from 525177 to 696569
 * Worker 5: IDs from 696569 to 867961
 * Worker 6: IDs from 867961 to 1039353
 * Worker 7: IDs from 1039353 to 1210745
 * Worker 8: IDs from 1210745 to 1382130
 *
 * Each worker then processes 5000 rows at a time
 * until they are done with their assigned work
 *
 *********************************************/

use \parallel\{Runtime, Future, Channel, Events};

$minId = 11001;
$maxId = 1382130;
$workers = 8;
$totalIds = $maxId - $minId;
// Try to divide IDs evenly across the number of workers
$batchSize = ceil($totalIds / $workers);
// The last batch gets whatever is left over
$lastBatch = $totalIds % $batchSize;
// The number of IDs (rows) to divide the overall
// task into sub-batches 
$rowsToFetch = 5000;

print "Total IDs: " . $totalIds . "\n";
print "Batch Size: " . $batchSize . "\n";
print "Last Batch: " . $lastBatch . "\n";

$producer = function(int $worker, int $startId, int $endId, int $fetchSize) {
    $tempMinId = $startId;
    $tempMaxId = $tempMinId + $fetchSize;
    $fetchCount = 1;
    
    print "Worker " . $worker . " working on IDs from " . $startId . " to " . $endId . "\n";
    
    while($tempMinId < $endId) {
        for($i = $tempMinId; $i < $tempMaxId; $i++) {
            $usleep = rand(500000, 1000000);
            usleep($usleep);
            print "Worker " . $worker .  " finished batch " . $fetchCount . " from ID " . $tempMinId . " to " . $tempMaxId . "\n";
            // Need to explicitly break out of the for loop once complete or else it will forever process only the first sub-batch
            break;
        }
        
        // Now we move on to the next sub-batch for this worker
        $tempMinId = $tempMaxId;
        $tempMaxId = $tempMinId + $fetchSize;
        if($tempMaxId > $endId) {
            $tempMaxId = $endId;
        }
        // Introduce some timing randomness
        $sleep = rand(1,5);
        sleep($sleep);
        $fetchCount++;
    }
    
    // This worker has completed their entire batch
    print "Worker " . $worker .  " finished\n";
    
};

// Create our workers and have them start working on their task
// In this case, it's a set of 171392 IDs to process
for($i = 0; $i < $workers; $i++) {
    $startId = $minId + ($i * $batchSize);
    $endId = $startId + $batchSize;
    if($i == ($workers - 1)) {
        $endId = $maxId;
    }
    \parallel\run($producer, array(($i+1), $startId, $endId, $rowsToFetch));
}

?>

Although function declaration is not allowed inside thread exec code, include is allowed. So if we want to declare a function, we could write another file that contain the function and include it.
# main.php
<?php
$runtime = new parallel\Runtime ();
$future = $runtime->run ( function () {
    $future = $runtime->run ( function () {
        include "included.php";
        return add (1, 3);
    }, [ ] );
echo $future->value ();
# output: 4
# included.php
<?php
function add($a, $b){
    return $a + $b;
}

<?php

/**
 * Sample parralel functional API
 * using a generator instead of a static list of items to process
 * 
 * Items to process in parallel come from a generator
 * It could be anything : e.g fetch a mysql array, a DirectoryIterator...
 * Thus the number of items to process in parallel is NOT known in advance
 * 
 * This algorithm attributes items to each parallel thread dynamically
 * As soon as a thread has finished working
 * It is assigned a new item to process
 * until all items are processed (generator closes)
 * 
 * In this example we process 50 items in 5 parallel threads
 * It produces output in this form (output changes at each run) :
 * 
 * ThreadId: 1 => Item: 1 (Start)
 * ThreadId: 2 => Item: 2 (Start)
 * ThreadId: 3 => Item: 3 (Start)
 * ThreadId: 4 => Item: 4 (Start)
 * ThreadId: 5 => Item: 5 (Start)
 * ThreadId: 5 => Item: 5 Sleep: 3s (End)
 * ThreadId: 5 => Item: 6 (Start)
 * ThreadId: 3 => Item: 3 Sleep: 4s (End)
 * ThreadId: 3 => Item: 7 (Start)
 * ThreadId: 2 => Item: 2 Sleep: 6s (End)
 * ThreadId: 2 => Item: 8 (Start)
 * ...
 * ThreadId: 4 => Item: 44 Sleep: 6s (End)
 * ThreadId: 4 => Item: 49 (Start)
 * ThreadId: 3 => Item: 46 Sleep: 5s (End)
 * ThreadId: 3 => Item: 50 (Start)
 * ThreadId: 2 => Item: 43 Sleep: 9s (End)
 * Destroy ThreadId: 2
 * ThreadId: 1 => Item: 47 Sleep: 5s (End)
 * Destroy ThreadId: 1
 * ThreadId: 4 => Item: 49 Sleep: 7s (End)
 * Destroy ThreadId: 4
 * ThreadId: 5 => Item: 48 Sleep: 10s (End)
 * Destroy ThreadId: 5
 * ThreadId: 3 => Item: 50 Sleep: 10s (End)
 * Destroy ThreadId: 3
 */

use \parallel\{Runtime, Future, Channel, Events};

// Generate list of items to process with a generator
function generator(int $item_count) {
    for ($i=1; $i <= $item_count; $i++) {
        yield $i;
    }
}

function testConcurrency(int $concurrency, int $item_count) {

    $generator = generator($item_count);

    // Function executing in each thread. Have a snap for a random time for example !
    $producer = function (int $item_id) {
        $seconds = rand(1, 10);
        sleep($seconds);
        return ['item_id' => $item_id, 'sleep_seconds' => $seconds];
    };

    // Fill up threads with initial 'inactive' state
    $threads = array_fill(1, $concurrency, ['is_active' => false]);

    while (true) {
        // Loop through threads until all threads are finished
        foreach ($threads as $thread_id => $thread) {
            if (!$thread['is_active'] and $generator->valid()) {
                // Thread is inactive and generator still has values : run something in the thread
                $item_id = $generator->current();
                $threads[$thread_id]['run'] = \parallel\run($producer, [$item_id]);
                echo "ThreadId: $thread_id => Item: $item_id (Start)\n";
                $threads[$thread_id]['is_active'] = true;
                $generator->next();
            } elseif (!isset($threads[$thread_id]['run'])) {
                // Destroy supplementary threads in case generator closes sooner than number of threads
                echo "Destroy ThreadId: $thread_id\n";
                unset($threads[$thread_id]);
            } elseif ($threads[$thread_id]['run']->done()) {
                // Thread finished. Get results
                $item = $threads[$thread_id]['run']->value();
                echo "ThreadId: $thread_id => Item: {$item['item_id']} Sleep: {$item['sleep_seconds']}s (End)\n";

                if (!$generator->valid()) {
                    // Generator is closed then destroy thread
                    echo "Destroy ThreadId: $thread_id\n";
                    unset($threads[$thread_id]);
                } else {
                    // Thread is ready to run again
                    $threads[$thread_id]['is_active'] = false;
                }
            }
        }

        // Escape loop when all threads are destroyed
        if (empty($threads)) break;
    }
}

$concurrency = 5;
$item_count = 50;

testConcurrency($concurrency, $item_count);

?>