pcntl_fork

<?php

$shm = shm_attach(ftok(__FILE__, 'p'), 1024, 0640);
shm_put_var($shm, 0, '');
$sem = sem_get(ftok(__FILE__, 'p'), 1, 0640, 1);

for ($i = 0; $i < 10; $i++) {
    if (0 === ($pid = pcntl_fork())) {
        $pid = posix_getpid();

        echo "Child $pid\n";

        // indicate to the parent that the child process has completed
        sem_acquire($sem);
        shm_put_var($shm, 0, str_replace("/$pid/", '', shm_get_var($shm, 0)));
        sem_release($sem);

        // kill the fork to avoid shutdown handlers
        posix_kill($pid, SIGKILL);
    }

    // indicate to the parent that a child process has started
    sem_acquire($sem);
    shm_put_var($shm, 0, shm_get_var($shm, 0) . "/$pid/");
    sem_release($sem);
}

while ('' !== ($pid = shm_get_var($shm, 0))) {
    echo "$pid\n";
    usleep(1);
}

sem_remove($sem);
shm_remove($shm);

echo "Done\n";