Warning!
The $sig parameter cannot be one of the PCNTL signals constants if you are running your script from the browser (ie. as an apache process). Strangely it does seem to allow the constants to be used from the command line ("php -r ...") but if your script is running from within apache then calling posix_kill($pid, SIGINT) will return FALSE (and posix_get_last_error() will return 0).