It depends on your reasons for learning. If it's for your own interest or to build personal sites then, yeah, a server-side language is greta cos then you can do a complete site alone. However if you are looking to work in the industry then I would pick one area to specialise in. Within web design agencies consisting of a decent number of staff, no site is completed by one person. You have front end developers, back end developers, information architects, designers, copywriters. If you're are working alone though, you have to do all of these on your own or outsource. It's good to have a grounding in everything though no matter what you want to do.
Also, as a side note, don't feel like you have to learn
PHP if you go for a server side language. There are many different ones around, all with their advantages and disadvantages. I am currently on a quest to understand Ruby. It's a more logical language than
PHP, I find, and it also has the added benefit of the Ruby on Rails framework. I've only just realsied how awesome Rails is.
Have a play, I say you learn them all... eventually.
Pete.
