The PHP client library is hosted at https://github.com/googleads/google-ads-php, and also available via dependency management systems:
The library now supports PHP version 7.1 or greater. It depends on gRPC and some other libraries, which you can see in the composer.json file. You will also need other information listed in Prerequisites.
To get OAuth2 refresh tokens, visit the following guides:
Visit the following guides for the library's features and utilities: