Description:
------------
When converting string from UTF-8 to UTF-8 by using mb_convert_encoding for 
replacing ill-formed byte sequence with the substitute character(U+FFFD), 
mb_convert_encoding replaces the character follwing ill-formed byte sequence with 
the substitute character. mb_convert_encoding also delete trailing ill-formed byte 
sequence and doesn't replace it with the substitute character.

The comprehensive test case for 2-4 byte 
characters is here: https://gist.github.com/masakielastic/5793665 .

Test script:
---------------
// U+24B62: "\xF0\xA4\xAD\xA2"
// ill-formed: "\xF0\xA4\xAD"
// U+FFFD: "\xEF\xBF\xBD"

$str = "\xF0\xA4\xAD".  "\xF0\xA4\xAD\xA2"."\xF0\xA4\xAD\xA2";
$expected = "\xEF\xBF\xBD"."\xF0\xA4\xAD\xA2"."\xF0\xA4\xAD\xA2";

$str2 = "\xF0\xA4\xAD\xA2"."\xF0\xA4\xAD\xA2"."\xF0\xA4\xAD";
$expected2 = "\xF0\xA4\xAD\xA2"."\xF0\xA4\xAD\xA2"."\xEF\xBF\xBD";

mb_substitute_character(0xFFFD);
var_dump(
    $expected === htmlspecialchars_decode(htmlspecialchars($str, ENT_SUBSTITUTE, 'UTF-8')),
    $expected2 === htmlspecialchars_decode(htmlspecialchars($str2, ENT_SUBSTITUTE, 'UTF-8')), 
    $expected === mb_convert_encoding($str, 'UTF-8', 'UTF-8'),
    $expected2 === mb_convert_encoding($str2, 'UTF-8', 'UTF-8')
);

Expected result:
----------------
bool(true)
bool(true)
bool(true)
bool(true)

Actual result:
--------------
bool(true)
bool(true)
bool(false)
bool(false)