PHP按长度准确截取中英文字符串

/* PHP截取中英文字符串，不按字符数而是按宽度来截取 * 仅针对UTF-8字符 */ <br><br>
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);
        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;
}