To successfully create UML diagrams with phUML you will need to install the graphviz toolkit.
phUML will use either
dot commands to create class diagrams.
How does it look like?
The image shown here is the class diagram generated by phUML when run on its original codebase. This image is hardly readable, because it has been resized to fit in the layout of this page. You can take a look at the complete image by clicking here.