Proof of Concept: Max Parallel execution in PHP, awaiting for all children to return before exiting.

max_parallelism.php

<?php

$maxChildren = 4;
$totalChildren = 20;
$pids = [];
echo "Max parallel children: {$maxChildren}\nTotal children: {$totalChildren}\n\n";

for ($i = 1; $i <= $totalChildren; $i++) {
    if (count($pids) >= $maxChildren) {
        $pid = pcntl_wait($status);
        unset($pids[$pid]);
    }
    echo "Giving Birth to Child $i\n";

    if (($pid = pcntl_fork()) < 0) {
        die('Could not give birth, perhaps we can adopt?');
    } elseif ($pid) {
        $pids[$pid] = $pid;
    } else {
        echo "I am alive! Child {$i}\n";
        sleep(rand(1, 10));
        echo "Time to go! Child {$i}\n";
        exit;
    }
}
echo "\n\nAll children has birthed, waiting for remaining children to live and pass away.\n\n";

while ($pids) {
    $pid = pcntl_wait($status);
    unset($pids[$pid]);
}

echo "\n\nThat was fun, cya :)\n";