I'm a website owner and I develop in Yii so in short, I think I'm working with more or less the "best" PHP has to offer (e.g. PDO, OOP...etc). My primary website (which is not built in any framework just native PHP) of 2.5 years is finally making enough money I can hire my first employee who I want to be a developer to convert it over to Yii. I've also started a new site which is just four weeks into development built with Yii but instead of building anything this week I'm going through RubyKoans because I keep wrestling with the idea that now there is no better time to convert to using Ruby on Rails...
I go through Yii and a lot of the most awesome stuff is ported from Rails (e.g. scopes). A lot of really great websites are built in rails (e.g. Twitter, GitHub, Hulu). Rails developers are "more" expensive but if you hire one are you not guaranteed a certain level of quality where as with PHP there's a lot of crummy devs out there? If I ever do make the switch, is it not better to do it now when it will be less painful? If I want to create a company that attracts talented developers, why not work in ruby so my company stands out?
At the same time PHP is fast, reliable, and when you're working with the "best" (e.g. Yii, Symfony2) I think it's a pretty nice and maintainable language. A lot of big sites are built in it too (Facebook, Digg, Craigslist, Hootsuite) plus all the porn sites which we all must admit deal with tons of traffic. Also there is way more PHP devs out there so as long as I use good interview practices will it not be easier to find some good PHP devs? Or would it be easier knowing every dev who came in for an interview already used ruby? Also a big part of me just wants to get started and not spend six months wading through ruby till I'm confident enough I want to port my existing projects over to it.
If you want to learn it, and you won't lose customers in the process, and you can see a good upside to the conversion - then go for it.
If you're doing it because you think there's some external market pressure to be in the "cool crowd", or that Ruby is inherently better than PHP - don't.
Languages all have their merits and demerits.
Language choice should only ever be a factor when you're looking to hire more people, or if you have a business reason to change platforms / development environments / etc.
Doing it just for the sake of doing it is dumb.
Are you providing development services to clients? If not, there's no compelling reason to move beyond what you have unless you feel there is a good business reason to do so.
I'm not going to get into a flame war on the merits / disadvantages of languages, but picking a language simply to be considered "up to date" isn't a good reason. Yii uses modern approaches / techniques - whats more important is what you offer your development staff, how you manage / don't manage them, and how you incorporate your customer development processes with software development.