Description:
------------
it does reproduce on "PHP8.0.3 + iconv 2.28 + Ubuntu 20.04",
it also reproduce on 3v4l.org everywhere from PHP5.6.0 to PHP8.0.9 inclusive, but unknown iconv/OS: https://3v4l.org/1b4G8

(the string also appears to trigger a bug in mb_check_encoding() that was fixed in 5.6.0?)

interestingly it does *not* reproduce on "PHP7.3.7-for-cygwin + iconv 1.0 + Windows 10", there it correctly strip the string down to string(3) "PHP"

it's possible that it's the result of a bug introduced sometime after iconv 1.0 and <= iconv 2.28?

Test script:
---------------
<?php
$invalid_utf8="\xF5\x80\x80\x80PHP";
$should_be_valid_utf8=iconv("UTF-8","UTF-8//IGNORE",$invalid_utf8);

var_dump($should_be_valid_utf8,bin2hex($should_be_valid_utf8),mb_check_encoding($should_be_valid_utf8));


Expected result:
----------------
string(3) "PHP"
string(6) "504850"
bool(true)


Actual result:
--------------
string(7) "����PHP"
string(14) "f5808080504850"
bool(false)