The Symfony blog has posted an article showing you how to prepare your applications for a migration to PHP 7 with the help of various polyfill libraries. These libraries make it possible to use PHP 7 functionality in non-PHP 7 applications if the function in use isn't defined.
According to the May 2017 PHP Stats, 53% of PHP developers use PHP 7.0 or 7.1, but only 10% of Composer packages require PHP 7.0 or higher. In fact, 1 in 4 packages still require PHP 5.3, which is used by less than 1% of developers.
[...] Upgrading your development machines is usually a simple task, but upgrading the rest of the infrastructure (servers, tools, etc.) usually requires more resources. This is where Symfony Polyfills can help you preparing the code of your application for PHP 7.
The article briefly explains what polyfills are and how to load in the current Symfony set via a Composer install. There've provided functionality for PHP versions 5.4 through 5.6 as well as PHP 7.0 and 7.1 to ensure you have the most up to date functionality at your fingertips.