DotNetOpenAuth programmatically without redirection

Question

I'm doing a simple project that consists of two separate parts:

1. Backend app in C# (ASP MVC) that server as business logic layer and exposes public controlers/actions with JSON responses
2. Frontend app in PHP that serves as an interface to the backend app

I have implemented Forms authentication with custom MembershipProvider which stores user details in Entity Framework model and SQL Server.

The next step would be to enable OpenID authentication. What I need to do is:

1. In my frontend get user OpenID credentials (probably Facebook and Google)
2. Forward those to my backend controler via POST request
3. Authenticate user with OpenID provider in my backend, storing their data (email only) in my model
4. Inform the frontend that user is authenticated

I've been going through different DotNetOpenAuth documents and samples, but I just can't figure out the point 3. in my list.