I can't attest to whether there's anything that ASP.Net can do that php can't... What I can say is that for web development ASP.Net is a very powerful language with a lot of built in functionality.
If your focus is web development, it never hurts to have more languages under your belt and more and more companies are looking for people who have ASP.Net skills.
I've done PHP professionally in a former job, but am now doing ASP.NET/C#. From a programmer standpoint I like it much better, as the language is strict, no variable types and such. In my opinion the change will indirectly improve your php coding too.
In addition to Lusipu advise, Here is mine,
To develop web applications in asp.net, you need to learn any of the programming languages C# or VB.NET and .NET object model (classes/components) specific to asp.net.
If you learn C# or VB.NET, you can also easily work in the following .NET based application platforms with little learning curve.
1. Desktop Applications (Windows Forms)
2. SharePoint Development
3. Microsoft Dynamics CRM
4. Microsoft BI platform
5. Windows Mobile Development
Also you can work BizTalk Server and Axapta Dynamics development etc . Becuase .NET Framework is the base for all Microsoft based platform customization. All you need to do is you have to learn the platform specific object model to work in that.
ASP.net is limited in the number of sites it will work on as it is Microsoft - Specific and there are relatively few MS servers,
However MS asp.net servers are often large sites and MS products ASP.net needs a lot of maintainence, and pay very well for that maintainence.
getting queries of-- 'I found this site in asp, can you make me a version that will run on my apache mysql php site'
and simply the more you know the more you can do