New on PHPMaster.com there's a tutorial from Lukas White about targeting users using geolocation based on the Geonames web service and a latitude/longitude. His example makes a call to find the closest "place" to the given coordinates.

Location-aware applications rely on being able to locate where you are, and this is what geolocation is all about. After all, once the application knows your location, it can go on to find the nearest store, guide you through the appropriate route to a destination, or target relevant advertisements to you. Geolocation, then, is simply the mechanism for identifying your geographical location.

He mentions two challenges associated with geolocation - finding where someone is and describing the location. With Geonames, he shows how to call the service's "findNearbyPlaceName" method to find the closest "place" to a given latitude/longitude combination. Included is a bit of sample PHP to connect to the service and Javascript to request the user's current location.