Use BOUML for "reverse engineering PHP code" [sic] to extract the class model (BOUML is available from "universe" repository of Ubuntu). I seriously recommend BOUML for this step because it's really fast compared to many other programs I have tried. In addition, it seems that BOUML seems to extract the model correctly (for the parts that BOUML even tries to extract).
If you are looking to generate UML easily from your existing PHP Classes you might want to consider PHPStorm 3.0 IDE. It does a good job of replicating existing code into UML.
phUML is fully automatic UML class diagramm generator written in PHP, licensed under the BSD license. It is capable of parsing any PHP5 object oriented source code and create an appropriate image representation of the oo structure based on the UML specification.