phUML is the best tool that I came across and worked only for me. You can easily create a class diagram from your existing code.
Go to the folder and try the following command to create class diagram in linux
./phuml -r /var/www/my_project -graphviz -createAssociations false -neato out.png
phUML - UML, PHP. PHP5-- UML.
./phuml --help