I get a lot of clients asking me about making mobile apps that connect to their websites to retrieve data, allow users to login, etc. Most of them have PHP-based sites, but don't have any clue about making APIs to interface with them. They ask me why I can't just connect directly to their SQL databases. I don't think that's a good thing to do from a mobile app. I would prefer they have some sort of API in place.
As far as PHP-based sites go, what are the best options when it comes to implementing an API for this purpose?