I need to add some new features to a PHP application. It is to follow the steps of a order. A process create some orders, the order goes to confirmation, then if approved is sent to a provider, later the provider confirm that can deliver the order, a request is made to the provider and so on...

I need to register when every step is made and send notifications. Also, some steps have a estimate time, and if that time is elapsed I need to send notifications so everybody know about the delay.

When a process starts, it have a predefined set of steps, but in a middle the user should be able to create new sub-steps, and delete or skip future steps..

Should I use a workflow engine? Which one do you suggests (free-opensource only)?