Well, it's not CI-friendly in the sense of being CI-aware, but I've used Notepad++ with the unofficial XDebug plug in. It's light and fast but a little buggy.
I'd like to mention CodeLobster, a free IDE that has a premium module that understands CI. I haven't bought that premium module, but I can still edit and debug CI files. I just don't get autocomplete.
To me, debugging support trumps everything else. But of course, I prefer free to paid. I code as a hobby, not as a profession.