If you learn PHP:
* You'll be able to see results running in your Web browser a lot quicker
* Your knowledge will be really relevant if you're looking to work on slightly older, more established Web sites like Facebook or Yahoo!, but
* You probably won't learn very good programming practices on your own - PHP has been pretty notorious for this, but maybe this has changed with PHP5, I haven't been keeping up.
If you learn Ruby:
* First you'll have to learn the language, and then you'll basically have to learn a framework like Sinatra or Rails before you'll be able to do awesome Web things
* Your knowledge will be really relevant to most startup-style Web sites begun since 2005 or so (e.g. Yelp, which is written in Python, which is basically sort of like a sister language to Ruby, give or take)
* You'll be much more likely to learn a lot of really good programming practices as you go that will serve you throughout your career.
Having worked with both PHP and Ruby, I'd recommend Ruby, because I think it's easier to learn, it's more fun, and it's a better investment; I'm pretty sure most HNers would agree if given the choice between those two languages, but your sample of respondents is probably pretty biased :)