function sub_str($str, $length = 0, $append = true)

{

    $str = trim($str);

    $strlength = strlen($str);

    if ($length == 0 || $length >= $strlength)

    {

        return $str;  //截取长度等于0或大于等于本字符串的长度，返回字符串本身

    }

    elseif ($length < 0)  //如果截取长度为负数

    {

        $length = $strlength + $length;//那么截取长度就等于字符串长度减去截取长度

        if ($length < 0)

        {

            $length = $strlength;//如果截取长度的绝对值大于字符串本身长度，则截取长度取字符串本身的长度

        }

    }

    if (function_exists('mb_substr'))

    {

        $newstr = mb_substr($str, 0, $length, EC_CHARSET);

    }

    elseif (function_exists('iconv_substr'))

    {

        $newstr = iconv_substr($str, 0, $length, EC_CHARSET);

    }

    else

    {

        //$newstr = trim_right(substr($str, 0, $length));

        $newstr = substr($str, 0, $length);

    }

    if ($append && $str != $newstr)

    {

        $newstr .= '...';

    }

    return $newstr;

}