How to make login feature

I am in the process of putting together a new website and I had some questions about development.I was curious if it was possible to take the login features from one site and just copy and paste those functions to a new site.Or do I have to rewrite everything from the beginning. I ask this because I am currently working for a website, but I want to create another site with many of the same features, but just a little bit different of a product. But we dont want to have to rewrite everything. Our developers says that it is not possible, but I have to believe that it is.What do you guys think