Phil Sturgeon has posted a set of best practices he follows when building out new APIs. It covers a wide range of topics at a high level, but it's a good checklist to get you started.

PHP developers are increasingly moving over to API development, as are a lot of server-side developers. It's a trend thats been happening for the last few years and it's getting to the point where everyone and their dog are putting articles showing off how to build "awesome" API's. Unfortunately most of these are either woefully inadequate or are promoting bad practices.

His recommendations include:

• Use the URI sparingly, and correctly
• Resources are EVERYTHING
• Authentication
• Background all the things
• Version your API like an adult

Each item in his list has some points to back it up and provide a bit more detail on what it's all about.