Php debugging would be awesome. There are NO! Linux editors that have php debugging. (well - there are some but they wicked heavy and written and java).
If someone has enough knowledge to get this done - I bet that geany will become the primary editor for many people.
I think this is better implemented with the GeanyGDB plugin. I actually have something like that already on my TODO list, but didn't find time yet to go deeper into this. However, patches welcome and so on.. :)