PHP バージョン 5.5 はコミュニティでサポートを終了したため、新しいアプリでは PHP 7 ランタイムを使用することを強くおすすめします。

Cloud Tasks REST API

The App Engine Task Queue REST API (v1), was turned down as of February 20, 2018. The separate product Cloud Tasks provides a REST API that you can use to add tasks from a second generation App Engine standard environment runtime, any of the App Engine flexible environment runtimes, or even from entirely outside of App Engine.

For first generation runtimes, the google\appengine\api\taskqueue package from the App Engine API is still available for use in your application.