Description:
------------
Iterating the characters of a UTF-32LE encoded string by using mb_substr() fails. Same string UTF-32BE encoded works as expected. 

Test script:
---------------
<?php

declare(encoding = 'UTF-8');
mb_internal_encoding('UTF-8');

header('Content-Type: text/plain; charset=UTF-32LE');

$string = "hällö wörld\n";
$string = mb_convert_encoding($string, "UTF-32LE");
$length = mb_strlen($string, "UTF-32LE");
echo mb_convert_encoding("Length: ". $length ."\n", "UTF-32LE");

echo $string;

for ($i=0; $i < $length; $i++) {
  echo mb_substr($string, $i, 1, "UTF-32LE");
}

?>

Expected result:
----------------
Length: 12
hällö wörld
hällö wörld

Actual result:
--------------
Length: 12
hällö wörld
häl