The [application_readable] field is specified in file [/home/{{your-project}}/phpMyAdmin/app.yaml]. This field is not configurable with runtime [php71{xx}] since static files are always readable by the application. It can safely be removed.

Ok, your app.yaml may look like this:

service: phpmyadmin
runtime: php55
api_version: 1

handlers:

- url: /(.+\.(ico|jpg|png|gif))$
  static_files: \1
  upload: (.+\.(ico|jpg|png|gif))$
  application_readable: true

- url: /(.+\.(htm|html|css|js))$
  static_files: \1
  upload: (.+\.(htm|html|css|js))$
  application_readable: true

- url: /(.+\.php)$
  script: \1
  login: admin

- url: /.*
  script: index.php
  login: admin

Solution:

• Remove application_readable: true