Description:
------------
xml_utf8_decode function incorrectly decode.

Reproduce code:
---------------
<?php
$ill=chr(0xf0).chr(0xc0).chr(0xc0).chr(0xa7);
$ill=addslashes($ill);
echo utf8_decode("$ill");
echo htmlspecialchars ($ill,ENT_QUOTES,"utf-8" );
?>

Expected result:
----------------
it will output a "'" incorrectly.

Actual result:
--------------
it will output a "'" incorrectly.

RTFM:
"This function decodes data , assumed to be UTF-8 encoded, to ISO-8859-
1."

chr(0xf0).chr(0xc0).chr(0xc0).chr(0xa7)

is not a valid utf8 data.

We know it is not, but the function only works on proper UTF-8 like is
documented. So there is no bug here, the bug is on your code calling
this function. You can't require functions to work like they're not
meant to work—but that's what you're asking here.