Ramiz,
Your title mentions "commerical web applications," while your question doesn't, so I'm assuming that the end goal is to write a web application using a functional programming language.
If this is the case, I recommend that you start with yaws, a fast HTTP server written in the functional concurrent programming language Erlang. You'll learn the ups and downs of twisting PHP-like scripts into a functional mindset, much of which you can then translate into your work with Haskell.
If you'd really like to stick with Haskell, and the choice of language is more important to you than the general functional approach, then take a look at the Real World Haskell book. That will give you a sense of how the language solves common problems that you might be interested in tackling while you learn the language.
