The four big web languages off the top of my head today would be: php, ruby, python, and javascript. You still see java in parts, or other jvm-based languages such as scala.

Personally I wouldn't worry much about which language you should learn. A lot of skill in programming and development is being able to conceptualize abstract problems, and learn new things on the go. I actually promote learning languages like haskell because, although you'll rarely use it professionally, it gets you thinking about things in different ways and, in turn, makes you a better programmer.

Just learn what you want for now (php?), and when you need or want to expand in the future, do so :).