PHP5 has no mb_trim(), so here's one I made. It work just as trim(), but with the added bonus of PCRE character classes (including, of course, all the useful Unicode ones such as \pZ).

Unlike other approaches that I've seen to this problem, I wanted to emulate the full functionality of trim() - in particular, the ability to customise the character list.

<?php
    /**
     * Trim characters from either (or both) ends of a string in a way that is
     * multibyte-friendly.
     *
     * Mostly, this behaves exactly like trim() would: for example supplying 'abc' as
     * the charlist will trim all 'a', 'b' and 'c' chars from the string, with, of
     * course, the added bonus that you can put unicode characters in the charlist.
     *
     * We are using a PCRE character-class to do the trimming in a unicode-aware
     * way, so we must escape ^, \, - and ] which have special meanings here.
     * As you would expect, a single \ in the charlist is interpretted as
     * "trim backslashes" (and duly escaped into a double-\ ). Under most circumstances
     * you can ignore this detail.
     *
     * As a bonus, however, we also allow PCRE special character-classes (such as '\s')
     * because they can be extremely useful when dealing with UCS. '\pZ', for example,
     * matches every 'separator' character defined in Unicode, including non-breaking
     * and zero-width spaces.
     *
     * It doesn't make sense to have two or more of the same character in a character
     * class, therefore we interpret a double \ in the character list to mean a
     * single \ in the regex, allowing you to safely mix normal characters with PCRE
     * special classes.
     *
     * *Be careful* when using this bonus feature, as PHP also interprets backslashes
     * as escape characters before they are even seen by the regex. Therefore, to
     * specify '\\s' in the regex (which will be converted to the special character
     * class '\s' for trimming), you will usually have to put *4* backslashes in the
     * PHP code - as you can see from the default value of $charlist.
     *
     * @param string 
     * @param charlist list of characters to remove from the ends of this string.
     * @param boolean trim the left?
     * @param boolean trim the right?
     * @return String
     */
    function mb_trim($string, $charlist='\\\\s', $ltrim=true, $rtrim=true)
    {
        $both_ends = $ltrim && $rtrim;

        $char_class_inner = preg_replace(
            array( '/[\^\-\]\\\]/S', '/\\\{4}/S' ),
            array( '\\\\\\0', '\\' ),
            $charlist
        );

        $work_horse = '[' . $char_class_inner . ']+';
        $ltrim && $left_pattern = '^' . $work_horse;
        $rtrim && $right_pattern = $work_horse . '$';

        if($both_ends)
        {
            $pattern_middle = $left_pattern . '|' . $right_pattern;
        }
        elseif($ltrim)
        {
            $pattern_middle = $left_pattern;
        }
        else
        {
            $pattern_middle = $right_pattern;
        }

        return preg_replace("/$pattern_middle/usSD", '', $string) );
    }
?>