I could be wrong, but I imagine that in order to accomplish this goal, I'll need to use Ruby on Rails and MooTools. You are wrong, in that your goal doesn't

I could be wrong, but I imagine that in order to accomplish this goal, I'll need to use Ruby on Rails and MooTools. You are wrong, in that your goal doesn't imply those requirements. RoR is one tool among hundreds that you could use to write a webapp. The same is true of MooTools. I'm not saying that they're not good options or even that they're not the best choices (maybe they are), but it doesn't follow from "I want to write a web-app on my mac" that "I need to learn RoR and MooTools". That being said. Yes, I think that an intelligent 23yo who knows php can probably learn Ruby (+ rails) and JS (+ MooTools). I'm not sure how useful an answer you can get here though. If people say yes, you're going to go and do it. If people say no, you're going to think to yourself "F--k that" and do it anyway (or at least you should).