Description:
------------
Commit: https://github.com/php/php-src/commit/ea491dba64d6c15a515e0f1cc58d881c2e2dc2f3#diff-a61ab806f8c2193851acdd3eb5f37232L489

Regarding bug: https://bugs.php.net/bug.php?id=39898

The above patch essentially removed the only usage of the constants FILTER_FLAG_SCHEME_REQUIRED and FILTER_FLAG_SCHEME_REQUIRED, however the constants themselves not only still exist, but the documentation's only hint about this is the changelog note: 5.2.1 FILTER_VALIDATE_URL now defaults to FILTER_FLAG_SCHEME_REQUIRED and FILTER_FLAG_HOST_REQUIRED.

The phrasing of this, and the continued presence of these constants, is misleading in that the word 'default' implies that it can be changed to something else, which it frankly cannot. Furthermore, there's no '_DEFAULT' flag against which one would expect to negate whatever defaults are supposedly enabled, hence the explicit 0 in the test script.

I suggest at least amending the doc page to better convey to users that the VALIDATE_URL filter *requires* both scheme and host since 5.2.1, and that those constants have no effect.

At some point I'd wager that these now-useless constants should be removed entirely, but doing so would be a potential BC break so I guess that would have to wait for PHP8.

Test script:
---------------
$no_scheme = '//google.com/foo/bar';
$no_host   = '/foo/bar';

var_dump(
    $no_scheme,
    parse_url($no_scheme),
    filter_var("$no_scheme",FILTER_VALIDATE_URL, 0)
);

var_dump(
    $no_host,
    parse_url($no_host),
    filter_var("$no_host",FILTER_VALIDATE_URL, 0)
);


Expected result:
----------------
No scheme:
string(20) "//google.com/foo/bar"
array(2) {
  ["host"]=>
  string(10) "google.com"
  ["path"]=>
  string(8) "/foo/bar"
}
bool(true)

No host:
string(8) "/foo/bar"
array(1) {
  ["path"]=>
  string(8) "/foo/bar"
}
bool(true)

// note: this would only be the expected result if the functionality matched the documentation.

Actual result:
--------------
No scheme:
string(20) "//google.com/foo/bar"
array(2) {
  ["host"]=>
  string(10) "google.com"
  ["path"]=>
  string(8) "/foo/bar"
}
bool(false)

No host:
string(8) "/foo/bar"
array(1) {
  ["path"]=>
  string(8) "/foo/bar"
}
bool(false)