Probably unnecessary check for Intl

[rimace]

The check and error message at DNSCheckValidation.php:38 seems unnecessary. Because if the extension doesn't exist, the function is provided by the package symfony/polyfill-intl-idn you defined as dependency in your composer.json.
As symfony/polyfill-intl-idn is a "hard" dependency and not just a suggestion, ! function_exists('idn_to_ascii') will never return true.