As PHP version 5.5 is no longer supported by the community, we strongly recommend new apps use the PHP 7 runtime.

Google App Engine PHP 5 Standard Environment documentation

The App Engine PHP 5 standard environment makes it easy to build and deploy an application that runs reliably under heavy load and with large amounts of data. Your application runs within its own secure, reliable environment that is independent of the hardware, operating system, or physical location of the server.