Mr. Composer, Jordi Boggiano, has posted an overview of composer features that have become available in the past year. A very nice one is config.platform.

The config.platform option lets you emulate which platform packages you have available on your prod environment. That way even if you have a more recent PHP version or are missing an extension locally for example, composer will always resolve packages assuming that you have the packages you declared installed.

That'll come in handy when you have a local php 7 environment and an older version installed on the server.