Let's go over those:

- Objective C: not a competitor as it doesn't do multi-platform, which I assume will be PHP's main angle.

- Javascript: weird OO model (compared to PHP's traditional OO), much smaller set of API's, typically tied to a web UI (PHP would probably wrap around native UI API's). Along with C# I consider this the strongest contender, but I can see why people would prefer PHP.

- Ruby, lua: does anyone actually use those on mobile? I'm sure they have their qualities, but those are really niche languages (on mobile). To me these are not serious competitors.