PHP - Troubles using pcntl_fork with shutdown function - php

Question

I had some several scripts that were running for a long time (6+ hours).
They were all containing a main loop that did it's thing, and a registered shutdown function that triggered a mysql query to announce the process as "done".
I decided to use pcntl_fork() inside these main loops, to run each round as a different process, in order to make the entire script complete faster.
It works OK, but, each child process is still registered with the shutdown function.
Therefore, each time a child process is complete it calls that mysql query and announce the script as complete.
How can I disable that shutdown function for the child processes, but keep it alive for the parent?
Sample code to understand what's going on:
common.php
register_shutdown_function('shutdown');
function shutdown()
{ global $objDb,$arg_id ;
echo "\n\n Executing queue process shutdown function.";
$objDb->query("UPDATE queue_args SET done='1' WHERE id='{$arg_id}'");
}
loop.php
include('common.php');
for ($i=1;$i<=200;$i++){
$pid = pcntl_fork();
if (!$pid) {
//child proccess - do something without calling the shutdown function
posix_kill(getmypid(),9);
}
} exit(); //this is when the shutdown function should eventually be called
Thanks

Answer 1

You could register the shutdown function within the if, like this:
if ($pid) {
if(!$registered) {
$registered = true;
register_shutdown_function('shutdown');
}
}else{
//child proccess - do something without calling the shutdown function
posix_kill(getmypid(),9);
}

Answer 2

You can't.
You could set a flag in the child processes after forking and poll it in the shutdown function - if it's set then return early. Or store the parent pid before you fork and, inside the shutdown function, return early if that's not the current pid. Or register the function in the parent process after forking.
if (!$pid) { posix_kill(getmypid(),9); }
This is really bad way to prevent the shutdown function from being invoked in the child process - but has all sorts of other implications - PHP won't shut down cleanly, buffers won't be flushed. Maybe you just need to do this:
#!/usr/bin/php
<?php
$arg_id = exec('task_which_forks.php');
exec("queue_clean_up.php $arg_id");