It's hard to find good/great developers period and when you find them language is rarely a concern. It's easier to find PHP developers though and dispite wanting to you'll never get to staff your team with just great developers once you grow over a couple people.
Say you have a job posting and you get 100 php developers and 5 Ruby ones. Of those 5 ruby ones 3 will people people who watched the screen casts. 2 will be intermediate/senior. Of the 100 php developers 50 of them will be horrible. 40 will be junior level. 10 will be a mix of intermediate to senior.
(In my experience anyhow, my company currently uses primarily php for services and RoR for control panels so we hire for either and cross train)
> It's hard to find good/great developers period and when you find them language is rarely a concern
Totally correct, however, with Ruby I think it's easier to find them primarily as there are less Ruby devs about, and using your example above, more or them are intermediate/senior (40% for Ruby, 10% for PHP). Then again intermediate/senior != good/great.
(Disclaimer: we're a PHP shop, but know a lot of really talented Rubyists)
From my experience, a good way of weeding out the 50 horrible php programmers, is to ask about (even simple) OOP.
Then to differentiate Junior from Senior just ask them what testing tools they use (testing is not [as] ingrained in the PHP community [yet]). If they can talk intelligently about PHPUnit, mocking, etc. they are more than likely senior level, or at least passionate enough to learn more than most PHP developers.