Write a Php function

I have a business site for multiple countries. I am trying to work on Php code which will automatically route users from US location to USA website and UK visitors to UK website. for example, a person from US types [login to view URL], then he should see a pop up, that currently you are accessing uk site. There should be a button to click where it will take user to USA site or select preferred country site. The script or code should be such that, If a business is launched in a new country then any non-technical person with little knowledge should be able to add new country in redirection list. Also I want someone who will perform changes while sharing screen as site contains sensitive data. Please furnish how will you implement this functionality, How much estimated hours will require. If you do this work with exactly given requirement and specification, then I will award more work. For a Php expert, this is a should not taken more than 2 hours.