Develop a PHP engine to analize a process described in XML

I have some process used in my company, described as an XML file, one for each process. Every process is composed by one to n tasks that are assigned to someone inside the company. I'll provide an XML file as example and the XSD file that describes the structure of such XML. I need a component (class), written in PHP that is able to parse these file and invoke actions on another object I'll provide, in order to make the process go ahed step by step. The methods are the createActivity, called once at the beginning of the process, and the createTask, to create any single task. Every time a task is closed, the system will use this PHP component to make the process go ahed to the next task(s), according to the schema described by the XML. The component will receive the XML as input and should return the same XML modified according the the progress made on the process. The read/write to the DB will be managed by the application. A standard PHP parser has to be used. Talking about the example file [login to view URL], "Section A" has to be completed, before "Section B" can start, while "area" elements inside a section can be executed at the same time. Task inside an area must be executed in sequence and, at the end of a task, it can be present a subprocess the follows the same rules as above. I need experienced PHP/XML developer; the component must be well tested ad I don't want to spend too much time in testing. Thank you.