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.
I'll second Ruby because of Sinatra and add some of my own thoughts. Sinatra is a very gentle introduction to separation of concerns. You can get a Sinatra app running on a shared Dreamhost with the same effort as a PHP site.
PHP was ok for me I spent a ton of time looking up functions in the docs but I could get stuff done. Once I had to edit/work with 3rd party code it was a nightmare.
I've developed in PHP, Ruby, C#, Java, ActionScript, and Python and Ruby's gem system is by far the easiest package management system I've used. Making your own gem is really easy.
Great ideas seem to pop up in Ruby first and then are ported to PHP, so why not use the original (see Rails, Cucumber, Bundler).
Here's a better idea. Take a very simple problem - say user fubar enters his name, you say "Welcome, fubar!", populate a backend database with his name because he's a new user. Otherwise you tell him "Welcome back, fubar!" and don't mess with your database.
This involves what ? Handling a GET, making a database insert, doing a select on name in a database, that's it.
Now go do this in atleast 5 frameworks, and time yourself.
PHP obviously, but do try RoR, Django, Play2, JSP and ASP.NET
I actually did the above exercise and came away with a lot of valuable insight. In my case Play2 was a breeze because on Heroku its trivial to set up, and each GET xxx request mapped directly to a xxx scala method in the Controller, so no magic. The others were a little more painful, but not a whole lot. I enjoyed the exercise.
The only trouble with this is that trivial applications tend to only give a superficial feel for the framework and aren't representative of a "real" development experience. For example, I wrote a trivial application in Flask. Python's Flask, while it looks great on the surface and has fantastic documentation, rapidly becomes unwieldy in the face of more complex problems. The same is true for Ruby on Rails.
Most frameworks almost feel like they're tuned to solve these basic problems and give a fantastic first impression that doesn't sustain.
Developing a complete project in each is a better way to move forward IMHO.
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.