Here's how I did it (directly from code to PDF drawing without manual drawing of anything):
Here's an example of suitable command line for using fdp to output PDF diagram (assuming that dot file generated by argouml-graphviz XLST processing is saved as xmi-model.dot):
fdp -Tpdf -Gmaxiter=1000 -Gmindist=0.5 -Gpackmode=node \
-Eweight=0.05 -Elen=1.0 -Eminlen=1.0 -Gsplines=true \
-Goverlap=false xmi-model.dot -oxmi-model.pdf
As an alternative you could try PHP_UML or php2xmi instead of BOUML for doing the "reverse engineering" part. I haven't yet tried that.
(I'm using the phrase "reverse engineering" because it seems that UML people are using those words when they mean extracting class and method information from the source code. I would personally interpret those words as extracting information from executable binary file or captured raw wire data.)
If you prefer drawing the class diagram by hand (instead of using computer to do all the drawing), you can use either BOUML or ArgoUML for the drawing. Using the "reverse engineered" data via BOUML will help in that case.