Last November I first released Emphloyer, a PHP library for asynchronous job processing with pluggable backends. Today I pushed out a small update to Emphloyer itself and the PDO backend to enable you to exert more control over what type of jobs are executed concurrently. In the new release you can specify a type for each of your jobs and then specify sets of employees that work on specific types or exclude specific types in your configuration files. Take a look at the the README for more information.
If you want to see an example of Emphloyer in action you can take a look at the emphloyer-example.
If you’re already using Emphloyer than please note that this update is not backwards compatible with the 0.1.x releases and that at the very least you will have to modify your configuration files (replacing the $numberOfEmployees setting with the $employees array).