Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

https://www.php.net/manual/en/filter.filters.validate.php
> Optional flag FILTER_FLAG_HOSTNAME adds ability to specifically validate
> hostnames (they must start with an alphanumeric character and contain only
> alphanumerics or hyphens).

Note that ⚡.kz must be converted to punycode, ie. xn--57h.kz. That would be considered valid with the HOSTNAME flag.

>Note that ⚡.kz must be converted to punycode, ie. xn--57h.kz. That would be considered valid with the HOSTNAME flag.

Thanks for your quick reply and clarification.

Just for future reference, the below code provides the expected result.

$domain_name = '⚡.kz';
$is_valid = filter_var(idn_to_ascii($domain_name), FILTER_VALIDATE_DOMAIN, FILTER_FLAG_HOSTNAME);
var_dump($is_valid);

Output

string(10) "xn--57h.kz"

Or don't use the HOSTNAME flag.

$domain_name = '⚡.kz';
$is_valid = filter_var($domain_name, FILTER_VALIDATE_DOMAIN);