We're planning a new project which will feature an API available to customers to interact with our app from their own web sites and systems. However never having built anything close to an full-fledged API in the past, I am not aware of any standards or recommendations that are available and could be used to make the API more easily adapted by our customers.
We will build the API using PHP and some parts of the API will need authentication while others don't. As of today, I have read a little about Oauth, SOAP & REST, but I have no idea on what is good practiec.
What technology/standard is recommended to base an API for 2011+ upon?