can't start googleappengine launcer .format(module_configuration.env, _ALLOWED_RUNTIMES_ENV_FLEX

Question

Following this tutorial https://cloud.google.com/php/tutorials/wordpress-app-engine-flexible

Every thing goes well. When i start to run googleAppEngineLauncer it throws this error in the last line of logs.

.format(module_configuration.env, _ALLOWED_RUNTIMES_ENV_FLEX)) google.appengine.tools.devappserver2.errors.InvalidAppConfigError: In env: flex, only the following runtimes are allowed: ('python-compat', 'java', 'java7', 'go', 'custom')

My app.yml look like this

runtime: php
env: flex

beta_settings:
cloud_sql_instances: myproject-id:europe-west1:wp

runtime_config:
document_root: wordpress

env_variables:
WHITELIST_FUNCTIONS: escapeshellarg,escapeshellcmd,exec,pclose,popen,shell_exec,phpversion,php_uname

I tried to use custom and php55 but still error.