Why is php file being downloaded (app.yaml )?

Question

PHP

app.yaml file

application: mydomain
version: 1
runtime: php55
api_version: 1
threadsafe: true
handlers:
- url:/
script: index.php

There is a file called test.php
when you enter mydomain.appspot.com/test.php
the file is simply downloaded(
How to make it work properly?