Davey Shafik has written an inspiring article for the International PHP Magazine:

"Whilst PHP is not the best tool for creating daemons, it does come fully loaded with the ability to fork, share memory, manage child processes etc. through the PCNTL, POSIX, Socket, SHMOP and System V Semaphore (sysvsem) extensions. I realise that I've just thrown five exotic extensions at most of you, but we're going to walk through them one by one just to familiarise you with each of them, what they are capable of, and what we're going to use them for."