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This RFC is superseded by RFC 3986, but the underscore is still in the unreserved characters: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

Start by reproducing the filter_var() calls with FILTER_VALIDATE_URL shown in the issue, then trace the URL validation implementation in php-src. Compare the handling of underscores in host and subdomain components with the cited RFC sections. Done means the documented examples produce the expected validation results and relevant tests cover the behavior.