PHP minor version update for September 2018

In September 2018, Azure App Service will update the PHP stacks to the latest available versions. For information on the changes in the new versions, see the change logs on the PHP website. | PHP version | Change log | | ----------- | -------------------------------------------- | | 5.6.37 | <https://www.php.net/ChangeLog-5.php#5.6.37> | | 7.0.31 | <https://www.php.net/ChangeLog-7.php#7.0.31> | | 7.1.20 | <https://www.php.net/ChangeLog-7.php#7.1.20> | | 7.2.8 | <https://www.php.net/ChangeLog-7.php#7.2.8> | * App Service * Features * SDK and Tools * [ App Service](https://azure.microsoft.com/en-gb/products/app-service/)