Ruby. It's the cleanest language with the most sensible standard library I've come across to date. Ruby doesn't necessarily mean Rails, and in fact I'm not a fan of Rails' overly prescriptive paradigm. (As an aside, any time you hear Rails is an MVC framework, discount it because it's really an MVA framework.) One of the things you'll find coming from the world of PHP is Ruby has consistent method signatures. For this and a lot of other reasons you'll almost certainly be happier with Ruby.

Aside from happiness, there are things that make Ruby more powerful. For example, you can cleanly implement the reactor pattern (think: NodeJS) with EventMachine and with EM-Synchrony you can avoid the callback hell that comes with NodeJS.

If you want, you can look at Sinatra to provide the common functionality that the PHP framework provides. With Sinatra you're not all-but-forced to use an ORM like you are with Rails, as Sinatra doesn't include one. If you really want, you can go closer to the metal and look directly at Rack, which is middleware between web servers (Apache, Nginx, etc) and the actual application.

There's really no right way to do it, but the best thing is to learn and explore. Some people will recommend Python, and while it's a decent language, it's got some things I consider to be rough edges -- lack of useful language features, such as switch/case statements, and badly implemented parts of its standard library that handle network requests.

The next language on my learning list is Go. I've heard nothing but good things about it, but I'll reserve my judgment until I have some experience with it.