php字串截取防止亂碼

/**
 +----------------------------------------------------------
 * 字串截取，支援中文和其他編碼
 +----------------------------------------------------------
 * @static
 * @access public
 +----------------------------------------------------------
 * @param string $str 需要轉換的字串
 * @param string $start 開始位置
 * @param string $length 截取長度
 * @param string $charset 編碼格式
 * @param string $suffix 截斷顯示字元
 +----------------------------------------------------------
 * @return string
 +----------------------------------------------------------
 */
function msubstr($str, $start, $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);
        if(false === $slice) {
            $slice = '';
        }
    }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;
}