Sorry, but that's just a bad idea. Good country-based ad targetting done in php will place a severe burden on your server.
Anyone could do it with relative ease - just use something like the
http://ip-to-country.webhosting.info/ database and you only need to write a small script - but you'd end up with something that just isn't suitable for a page with heavy traffic.
You should seek contact with one of the companies that already have a script in C that detects country, and ask them how much some custom work would cost.