Bence Eros has submitted a new post from his blog that looks at the method he came up with to create an interactive debugger for PHP. It's just a prototype/proof of concept that lets you execute PHP code in the browser (using eval - a little scary, I'll admit).
Last night I created a prototype for an interactive debugger for PHP without the need of any IDE-plugin. In this post I'm going to show what I found. Unfortunately you will need some time to put the environment together, but I think it's worth doing it. My target was to create a way how you can view and modify your variables manually at runtime, only by inserting one line code.
He does mention that, because of the settings he uses, the debugger can only be loaded from a different domain than the application which can be restricted a bit simpler (i.e. its own https or .htauth). His setup uses an Apache2 web server and a few PHP/HTML scripts to get the job done. In the end you'll have something similar to this for you to run your code in.