My boss wants me to make a web application in .NET, it's like asking a Android developer to build the app for iOS as well.
Note: Im a PHP developer

If you have the bandwidth .NET will be an opportunity. Your PHP knowledge will get you up-to-speed real fast. I just evaluated two very expensive enterprise solutions written in ASP.NET. One excellent and one that sucks. Point is there is opportunity for talent and corporations will pay big bucks. What do you think?

If you have the capacity to expand your skill set then go for it.

Totally agree with all the above statements about expanding your skill set, you won't regret it.

On the other side, regardless of expanding your skillset try to understand why .NET over PHP in this situation. The cost ramifications of learning a new language for no reason can be astronomical.