PHP spawn background process with a specific process ID range

Question

I'm working on a PHP function that spawns PHP background processes that do something, the issue I'm facing is that my script is potentially picking a process ID that's already being used on the system, my function, shell_exec needs to pick from a range, so for example, only pick a process ID between 3000 - 4000, meaning a possible 1000 different process IDs that it can choose from, how can I achieve this with my current function:

// trigger the process
$bgProcessID = shell_exec(sprintf(
    '%s > %s 2>&1 & echo $!', $this->getQueueProcessCommand($jobID), '/dev/null'
));

For reference, but unrelated, getQueueProcessCommand is:

protected function getQueueProcessCommand($job)
{
    $appDir = rtrim(APP, '/');
    return "$appDir/Console/cake queue_manager process --job=$job";
}