asynchronous processing with PHP - one worker per job

Well, if you're on Linux, you can use pcntl_fork to fork children off. The "master" then watches the children. Each child completes its task and then exists normally.

Personally, in my implementations I've never needed a message queue. I simply used an array in the "master" with locks. When a child got a job, it would write a lock file with the job id number. The master would then wait until that child exited. If the lock file still exists after the child exited, then I know the task wasn't completed, and re-launch a child with the same job (after removing the lock file). Depending on your situation, you could implement the queue in a simple database table. Insert jobs in the table, and check the table in the master every 30 or 60 seconds for new jobs. Then only delete them from the table once the child is finished (and the child removed the lock file). This would have issues if you had more than one "master" running at a time, but you could implement a global "master pid file" to detect and prevent multiple instances...

And I would not suggest forking with FastCGI. It can result in some very obscure problems since the environment is meant to persist. Instead, use CGI if you must have it web interface, but ideally use a CLI app (a deamon). To interface with the master from other processes, you can either use sockets for TCP communication, or create a FIFO file for communication.

As for detecting hung workers, you could implement a "heart-beat" system, where the child issues a SIG_USR1 to the master process every so many seconds. Then if you haven't heard from the child in two or three times that time, it may be hung. But the thing is since PHP isn't multi-threaded, you can't tell if a child is hung or if it's just waiting on a blocking resource (like a database call)... As for implementing the "heart-beat", you could use a tick function to automate the heart-beat (but keep in mind, blocking calls still won't execute)...

The workerpool might be interesting:

https://github.com/qxsch/WorkerPool

https://github.com/qxsch/WorkerPool/blob/master/examples/asyncExample.php

while you do run asynchronous one task with many job with pcntl_fork or you will creating persistence query every (s)seconds, be carefully with high cpu consumption, you can get hanging processing memory because can't be allocated memory again, i think best choice you can build fully with Gearman, or you can try with cloud worker such as IronWorker.