php7cc is a command line tool designed to make migration from PHP 5.3-5.6 to PHP 7 easier. It searches for potentially troublesome statements in existing code and generates reports containing file names, line numbers and short problem descriptions. It does not automatically fix code to work with the new PHP version.

You can learn more about this in PHP benchmarks. This means that there is an issue with backwards compatibility if you implement PHP 8.0 union types, as this will break on sites running PHP 7.4 or below.

The team behind the release of PHP 8 boasts in their announcement of better performance, better syntax, and improved type safety. PHP 8 is faster than PHP 7, the syntax requires less code, and typing is better supported. These three improvement categories provide a powerful major version release of PHP.