Proximity search using codeigniter

we have a database of cities, thats shows in the dropdown. You need to write a script such that the visitor's current city is preselected in the dropdown of our sign up form and if their city is not listed in our db, nearest city shows up. It will done using maxmind geoip database. We know how it will be done, you just have to code it. job must be completed today, will pay $50 for the job. Site is designed in codeigniter.