API v2
In mid-2017 we've reworked the API for fetching content and released the results under the name Content Query API v2. The new API has been simplified in terms of the content structure — in particular to rely less on development kits.
All the differences between API v1 and v2 are documented in a separate article.
We plan to continue supporting both v1 and v2, as we have no intention of making anyone upset by breaking their well-crafted projects.
PHP starter kit v2
The API update had to be reflected in the development kits that we provide. After releasing the JS starter kit v2 we reworked the PHP starter kit.
What do I need to do with my old PHP projects that use API v1?
You don't need to do anything. The API v1 will be supported and your projects will be just fine.
We recommend that you use the new starter kit for your new projects, though, for better performance and general peace of mind.