自己写的php中文截取函数mb_strlen和mb_substr，_PHP教程

/* from Internet, author unknown */
if (!function_exists('mb_substr')) {
    function mb_substr($str, $start, $len = '', $encoding="UTF-8"){
        $limit = strlen($str);
 
        for ($s = 0; $start > 0;--$start) {// found the real start
            if ($s >= $limit)
                break;
 
            if ($str[$s]                 ++$s;
            else {
                ++$s; // skip length
 
                while ($str[$s] >= "\x80" && $str[$s]                     ++$s;
            }
        }
 
        if ($len == '')
            return substr($str, $s);
        else
            for ($e = $s; $len > 0; --$len) {//found the real end
                if ($e >= $limit)
                    break;
 
                if ($str[$e]                     ++$e;
                else {
                    ++$e;//skip length
 
                    while ($str[$e] >= "\x80" && $str[$e]                         ++$e;
                }
            }
 
        return substr($str, $s, $e - $s);
    }
}