php_fork.txt

Example script that uses pcntl_fork() to fork itself a few dozen times, and System V IPC functions to create a message queue,
  let the child processes send data to it and the parent process read and display it.

#!/usr/bin/env php
<?php
// Create or attach to a message queue
$queue = msg_get_queue(ftok(__FILE__, 'a'), 0600); # set permissions so that only the owner can read/write to this queue
# ftok() creates a message queue key based on the file's inode and a single character project identifier. The latter lets
# one create several queues all based on/in the same file.

$child_count = 50;

// Fork child processes
for ($i = 0; $i < $child_count; $i++) {
    \e[36m# pcntl_fork() splits the script \e[1mat the point where it is called\e[0;36m. The two copies of the script know what they are
    \e[36m# based on the value of $pid: 0 for the child, positive int for the parent (it will be the child's PID) or -1 for failure.\e[0m
    $pid = pcntl_fork();
    if ($pid == -1) {
        die('Could not fork');
    } elseif ($pid) {
        \e[36m# Parent process\e[0m
        // Do nothing, will wait for child messages later
    } else {
        \e[36m# Child process: do all the Child things, send the results to the queue, then \e[1mexit\e[0m
        $sleepTime = rand(1, 5);
        sleep($sleepTime);
        
        \e[36m# Send a message to the parent\e[0m
        msg_send($queue, 1, "Child $i slept for $sleepTime seconds");
        
        exit(0);
    }
}

# Parent process waits to receive messages from all child processes
for ($i = 0; $i < $child_count; $i++) {
    msg_receive($queue, 0, $msgType, 1024, $message, true);
    echo "Received message: $message\\n";
}

# Destroy the message queue, otherwise it hangs around in memory. \e[38;5;208mipcs\e[0m will show you current message queues.
msg_remove_queue($queue);
# You might be tempted to use register_shutdown_function() for this (ChatGPT told me to!) but that will also fire as soon as
# the first child process exits, sooo.. don't.

# If all else fails, from the cli use \e[38;5;208mipcrm -q <msqid>\e[0m, where the msqid shows up under the output of \e[38;5;208mipcs\e[0m.

# Other considerations:
# - make sure the child process Always exits, and always sends something to the queue before doing so, wrapping it in a try/catch
#   should work
# - counting $children++ inside the child process does not work, this variable will not be incremented in the parent process.
#   you can increment it in the parent branch of the $pid fork.