The Google App Engine PHP SDK is provided for developers that are building libraries designed to compose with the App Engine SDK.
This is a subset of the full App Engine SDK that is available for developers that are simply writing applications to run on App Engine. You can download the full SDK for developing applications from here.
Please see API documentation at https://googlecloudplatform.github.io/appengine-php-sdk/2.0.0. Or more generally, https://googlecloudplatform.github.io/appengine-php-sdk/"release-tag-number".
This Gen 2 SDK is currently in private preview. To gain access, please sign up at https://docs.google.com/forms/d/e/1FAIpQLSd1hFLA2UFSYwIMxm9ZI3pwigORZBgjJRH0qrnhtE7nvhhRCQ/viewform.