Sebastian Bergmann - eZ Components Workflow Engine:

" The Workflow components that I developed as part of my Diploma thesis ("Design and Implementation of an Activity-Based Workflow Engine") and that will be part of the eZ Components, an enterprise ready general purpose PHP components library by eZ Systems, provide this layer in the form of an abstract virtual machine for Graph-Oriented Programming (GOP) with PHP.

[...] You can find the sourcecode for the three components that I developed as part of my thesis here: Workflow, WorkflowDatabaseTiein, and WorkflowEventLogTiein."