Ok,
it goes this way:
At the projects tab (top left of the screen in my configuration), right click on the name of the project, and then properties => run configuration
There you have to fill 3 forms:
1. project url - the base url of the project on web, typically http://localhost
2. index file - use the browse button and go to the local copy of index.php or whatever file serves this role for you. This form should contain the path to this file in the local filesystem , not on the web
3. arguments - any arguments you wish to path to your script through the URL
This should give you the ability to choose a custom URL for debugging
A possible problem: in this process you may loose the synchronization between web addresses and the corresponding files in your system. Google "path mapping in php debugger" if you encounter issues of this nature, and take the first result (which the system wouldn't allow me to link directly).
However this didn't work for me. I used server configuration to overcome this issue by redirecting URLs
Gidi
p.s.
All the above doesn't explain how to pass POST variables to your script while debugging (and I indeed don't know how to do it)