A couple of months ago I wrote about console commands that create a PID file containing the process id (PID) of the PHP process that runs the script. It is very usual to create such a PID file when the process forks itself and thereby creates a daemon which could in theory run forever in the background, with no access to terminal input or output. The process that started the command can take the PID from the PID file and use it to determine whether or not the daemon is still running.