I have tried using $php -l and couple other tools. However the best one in my experience (YMMV, of course) is scheck of pfff toolset. I heard about pfff on Quora (http://www.quora.com/Is-there-a-good-PHP-lint-static-analysis-tool)
You can compile and install it. There are no nice packages (on my mint Debian, I had to install libpcre3-dev, ocaml, libcairo-dev, libgtk-3-dev and libgimp2.0-dev dependencies first) but it should be worth an intsall.
The results are reported like
rjha@mint ~ $ ~/sw/pfff/scheck ~/code/github/sc/
login-now.php:7:4: CHECK: Unused Local variable $title
go-automatic.php:14:77: CHECK: Use of undeclared variable $goUrl.
