The Geocodio PHP Sample Code demonstrates how to use Geocodio API endpoints to geocode a single address into coordinates, reverse geocode a single coordinates into addresses, batch geocode up to 10,000 addresses, and batch reverse geocode up to 10,000 coordinates using PHP code.
Your next geo app is going to be easier to build. Instead of running your own database and collecting geographic data yourself, you can build on top of TownMe and its newly-launched read/write Geo API (tech details at our TownMe API profile).
Back in February 2010 we asked Why can't you share photos on Foursquare? We discussed that this could be seen as an oversight or an opportunity for others. However, this question has now been deprecated by Foursquare with the introduction of its add photo API method, which suggests that the previous absence of a method was simply because it wasn't a priority, it was an oversight or the opportunity hasn't been taken up by developers as Foursquare had hoped. In addition to the new add photo method, Foursquare have also introduced API methods which allow comments to be added and deleted from checkins. These methods have already been rolled into the official iPhone and Android apps along with a few 3rd party apps that use the Foursquare API.