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<?php $domain = 'a-valid-domain.com'; var_dump(filter_var($domain, FILTER_VALIDATE_DOMAIN));
based on ZiE3O
Output for 7.2.0 - 7.2.34, 7.3.0 - 7.3.26, 7.4.0 - 7.4.14, 8.0.0 - 8.0.1
string(18) "a-valid-domain.com"