tl;dr – The canary process for the 1.9.17 PHP Runtime has identified an issue with the Google APIs Client Library for PHP that will cause applications to break. Update your version to at least 1.1.2 ASAP.
During the canary process for the new version of the PHP runtime, an issue has been uncovered related to the Google API PHP client and the cURL extension. The App Engine sockets library does not allow socket connections to www.googleapis.com, which prevents the client library from working if it uses cURL, which is it’s default configuration.
We have made a change to the client library so that it will always use PHP streams on App Engine, which prevents this error from occurring. However this means that your application code will need to be updated to the new version of the client library and the code re-deployed to the production environment.
To reduce the possibility of an outage for your application we encourage you to update this library and re-deploy your application as soon as possible.
In the meantime we continue to assess the number of affected applications to determine what the best course of action is to ensure minimal disruption for existing apps. This may well mean delaying the release of the PHP 5.5 runtime with cURL support to allow sufficient time for applications to be upgraded.
Update 28 Nov: We have decided to roll back the 1.9.17 PHP release with PHP 5.5 and cURL for the time being. You have plenty of time to update your applications to the latest API client.