On the ReviewSignal blog today there's a post looking at their use of long running PHP processes and how they got around some of the common problems.
Here at Review Signal, I use a lot of PHP code and one of the challenges is getting PHP to run for long periods of time. Here are two sample problems that I deal with at Review Signal that require PHP processes to run for long periods of time or indefinitely: data processing and Twitter streaming API data.
They talk some about their use of the PHP CLI, bash and cron to execute their scripts. There's a bit about the difference between executing a script in ssh versus cron and how to use "nohup" to have it execute in the background. They show how to set up a cron job and, more specifically, how their script pulls from the Twitter API via a bash script to check if it's already running.