I don't know about PHP going away anytime soon. It runs huge amount of websites based on the LAMP (Linux, Apache, MySql, PHP) model. Also it is free.
Anyway, to answer your question, C# can be used to develop console (windows applications), web applications, and new platforms like Silverlight that have a foothold in both web and PC environments.
If you are a student, Microsoft will GIVE you a copy of their professional software. Go to dreamspark.com to see if you qualify. If you aren't student, you can still start for free with MS Express software development IDE, Visual Web Developer. Download at
http://www.microsoft.com/express/vwd/
There are, dare I say, millions of websites using C# and ASP.NET? I program websites all day using C# and ASP.NET. There are many advantages and a few disadvantages of using ASP.NET. It is certainly taking over ColdFusion in many companies. Many companies are consolidating on MS platforms:
Windows
SharePoint
SQL Server
and knowing C# will allow you to communicate with all those products.
There are many books on C#. Jesse Liberty has a good reputation for decent books. The "Head First" series is also nice, if you like their writing style. Also, I like books from Manning publisher.
You can learn a huge lots from the Microsoft websites for ASP.NET
http://asp.net
Good luck