Is there a interactive debugger for php like ruby's debugger?

I watched the Creating a weblog in 15 minutes with Rails 2 and after 9 minutes in the video he shows ruby's interactive debugger, which allows you to call functions/methods from within a running script.

This goes way beyond breakpoints and looks very useful.

Is there something for PHP that gives similar functionality?

From stackoverflow

• Install xdebug and then use one of the debug clients mentioned here.