koriym/print_o is an object graph visualizer for PHP. Here’s a Wikipedia answer to the question of “What is an object graph?”:
Object-oriented applications contain complex webs of interrelated objects. Objects are linked to each other by one object either owning or containing another object or holding a reference to another object. This web of objects is called an object graph and it is the more abstract structure that can be used in discussing an application’s state.
This tool is similar to some of my GraphViz tools (CakePHP model visualization with GraphViz, and PHP class inheritance with GraphViz), but it’s a lot more generic, and looks like a lot more powerful.