Mixpanel PHP-SDK currently uses PHPUnit 5.6.*. This version is incompatible with PHP 7.4 or higher. It is recommended to migrate PHPUnit to a current version, at least PHPUnit 8, better would be version 10. https://phpunit.de/supported-versions.html
This should only be relevant for development and should not break anything from the usage perspective.
If this is wanted, I would be open to create a PR for that. Please let me know.