Match users to their geo-location with PHP and PEAR Net:Geo

location based services

Nice article on location based services and how to implement them for example with PHP and the correspondig PEAR package:
Ever wondered how sites like Google and Yahoo! always seem to know which country you’re in, and automatically redirect you to the most appropriate country sub-site? Or how some Web sites display advertisements perfectly targeted to your city or region? Well, it isn’t magic — these sites are simply mapping your IP address to a geographical database to identify your location, and then using this information to send you precisely-targeted data.
http://builder.com.com/5100-6371_14-6103179.html?part=rss&subj=bldr