一个php写的截取字符串的函数

/**
     * 字符串截取，支持中文和其他编码
     * @static
     * @access public
     * @param string $str 需要转换的字符串
     * @param string $start 开始位置
     * @param string $length 截取长度
     * @param string $charset 编码格式
     * @param string $suffix 截断显示字符
     * @return string
     */
    static public function msubstr($str, $start=0, $length, $charset="utf-8", $suffix=true) {
        if(function_exists("mb_substr"))
            $slice = mb_substr($str, $start, $length, $charset);
        elseif(function_exists('iconv_substr')) {
            $slice = iconv_substr($str,$start,$length,$charset);
        }else{
            $re['utf-8']   = "/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|[\xe0-\xef][\x80-\xbf]{2}|[\xf0-\xff][\x80-\xbf]{3}/";
            $re['gb2312'] = "/[\x01-\x7f]|[\xb0-\xf7][\xa0-\xfe]/";
            $re['gbk']    = "/[\x01-\x7f]|[\x81-\xfe][\x40-\xfe]/";
            $re['big5']   = "/[\x01-\x7f]|[\x81-\xfe]([\x40-\x7e]|\xa1-\xfe])/";
            preg_match_all($re[$charset], $str, $match);
            $slice = join("",array_slice($match[0], $start, $length));
        }
        return $suffix ? $slice.'.' : $slice;
    }