PHPCompatibility 9.3.5 was released on December 27, 2019 and PHP 8.0.0 was released on November 26, 2020.

As a consequence using PHPCompatibility 9.3.5 has lots of issues with PHP 8 code, so it would seem wise to suggest to use the development branch of PHPCompatibility in the README.

I can prepare a pull request if wanted.