I come from a PHP background and I am going ahead with building a Windows Store App.
I am wondering how would I come across storing data in the cloud on a windows app so if my app requires a user to register and login to access the app how would I store it in a database?
I understand with PHP it's just setting up the connection and using SQL queries to store and retrieve however what is the approach with Windows Apps?
For Windows Store Apps (both phone and desktop/tablet, truly universal apps aren't available yet) you store data in the cloud the same way you usually would:
Use a web service
That can be a PHP page if you want. Since you are in C#, the recommended way would be to host a service on Microsoft Azure (which can also host your database). You can also host a WCF service on any compatible platform and use that.
That said, any service that you can access will work, and .NET for Windows Store (aka the Windows App API) is pretty flexible in what it will let you do (services, sockets, HTTP methods, etc.).