How is it possible to use Phong Shading (as an extension or otherwise)?
I think there is no extension for the phong shading.
I do know that about half a year the opengl.org site published a code that implements the phong shading, if it is important for you - try searching it…
Never heard about actual implementation of GL_WIN_phong_shading.
You can use multipass rendering with environment maps to approximate it.
or try perpixel lighting demos from NVIDIA’s dev. site - Phong shading is very similar to dot-product bumpmaping (but without bumps ).