FILTER_VALIDATE_URL returns false when underscore present in URL

Description

The following code:

<?php
var_dump(filter_var('https://sub_domain.example.com', FILTER_VALIDATE_URL));
var_dump(filter_var('https://ex_ample.com', FILTER_VALIDATE_URL));

Resulted in this output:

bool(false)
bool(false)

But I expected this output instead:

string(30) "https://sub_domain.example.com"
string(20) "https://ex_ample.com"

The underscore is a valid character according to the RFC 2396 section 2.3:

Unreserved Characters

Data characters that are allowed in a URI but do not have a reserved
purpose are called unreserved. These include upper and lower case
letters, decimal digits, and a limited set of punctuation marks and
symbols.

  unreserved  = alphanum | mark

  mark        = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"

Unreserved characters can be escaped without changing the semantics
of the URI, but this should not be done unless the URI is being used
in a context that does not allow the unescaped character to appear.

But this filter fails if a underscore is present in the domain or subdomain portion of the URL.

This RFC is superseded by RFC 3986, but the underscore is still in the unreserved characters:

unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"

PHP Version

PHP 8.4.4

Operating System

No response

まず、Issue に示されている FILTER_VALIDATE_URL を指定した filter_var() の呼び出しを再現し、その後 php-src にある URL 検証の実装を追ってください。ホストおよびサブドメインの各コンポーネントにおけるアンダースコアの扱いを、引用されている RFC のセクションと比較してください。文書化された例が期待される検証結果を生成し、関連するテストでこの挙動がカバーされれば完了です。