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)