The function trim() has not failed me so far in my multibyte applications, but in case one needs a truly multibyte function, here it is. The nice thing is that the character to remove can be whitespace or any other specified character, even a multibyte character.

<?php

// multibyte string split

function mbStringToArray ($str) {
    if (empty($str)) return false;
    $len = mb_strlen($str);
    $array = array();
    for ($i = 0; $i < $len; $i++) {
        $array[] = mb_substr($str, $i, 1);
        }
    return $array;
    }

// removes $rem at both ends

function mb_trim ($str, $rem = ' ') {
    if (empty($str)) return false;
    // convert to array
    $arr = mbStringToArray($str);
    $len = count($arr);
    // left side
    for ($i = 0; $i < $len; $i++) {
        if ($arr[$i] === $rem) $arr[$i] = '';
        else break;
        }
    // right side
    for ($i = $len-1; $i >= 0; $i--) {
        if ($arr[$i] === $rem) $arr[$i] = '';
        else break;
        }
    // convert to string
    return implode ('', $arr);
    }

?>