Tony Marston - An activity based Workflow Engine for PHP:

“This document will describe the activity based workflow system which I have constructed as an extension to my Development Infrastructure for PHP. […]

In order to implement a workflow system it is first necessary to find a suitable means of designing and modeling a workflow process. For this I take advantage of the work done by Carl Adam Petri who was the first to formulate a general theory for discrete parallel systems which gave birth to what are now known as Petri Nets.”