Description:
------------
There is a problem with mbfl library functions. UTF-16, UTF-16BE, UTF-16LE encodings are parsed by the slow path (variable width multibyte encodings). 


Test script:
---------------
<?php
function bmark()
{
    $args = func_get_args();
    $len = count($args);

    if ($len < 3) {
        trigger_error("At least 3 args expected. Only $len given.", 256);
        return false;
    }

    $cnt = array_shift($args);
    $fun = array_shift($args);

    $start = microtime(true);
    $i = 0;
    while ($i < $cnt) {
        $i++;
        $res = call_user_func_array($fun, $args);
    }
    $end = microtime(true) - $start;
    return $end;
}

$i = 1000;
$str32 = mb_convert_encoding(str_repeat("строка", $i), "UTF-32", "UTF-8");
$str16 = mb_convert_encoding(str_repeat("строка", $i), "UTF-16", "UTF-8");

echo "str16: " . bmark($i, "mb_substr", $str16, 0, null, "UTF-16") . PHP_EOL;
echo "str32: " . bmark($i, "mb_substr", $str32, 0, null, "UTF-32") . PHP_EOL;

?>

Expected result:
----------------
UTF-16 and UTF-32 parsing speed should be mostly the same. 

Actual result:
--------------
str16: 0.13957190513611
str32: 0.026632070541382