PHP字符串截取

if (!function_exists('sub_str')) {
    /**
     * 字符串截取
     * @param string $str 需要截取的字符串
     * @param int $start 开始位置
     * @param int $length 截取长度
     * @param bool $suffix 截断显示字符
     * @param string $charset 编码格式
     * @return string 返回结果
     */
    function sub_str($str, $start = 0, $length = 10, $suffix = true, $charset = "utf-8")
    {
        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));
        }
        $omit = mb_strlen($str) >= $length ? '...' : '';
        return $suffix ? $slice . $omit : $slice;
    }
}