As Heroku points out in their latest blog post the popularity of PHP 7 is already rising quickly since they made this major release available on their service and how it's now reached the point of being the most deployed version on their systems.

Last week, we made PHP 7 available on top of a new, reworked version of our PHP support, and our users are adopting PHP 7’s exciting new features and stellar performance improvements quickly - we’re already seeing PHP 7 being used in the majority of PHP deploys on Heroku.

Under the hood, much of the logic that handles a deploy has changed, but not the fundamental principles upon which our support for PHP applications is designed. We've always been determined to provide a fully standards-based implementation that does not rely on proprietary configuration or magic behavior, and that means building on top of a great foundation of tools.

They also mention the rise of Composer in recent years in the PHP community and how they're continuing to treat the composer.json (and .lock) and installation process as first class citizens, making use of Composer itself to install not only dependencies but also language runtimes and extensions.