/**

+----------------------------------------------------------

 * 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

+----------------------------------------------------------

 */

function msubstr($str, $start=0, $length, $charset="utf-8", $suffix=true)

{

    if(function_exists("mb_substr"))

        return mb_substr($str, $start, $length, $charset);

    elseif(function_exists('iconv_substr')) {

        return iconv_substr($str,$start,$length,$charset);

    }

    $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, $match);
//通过preg_match_all将$str字符串进行编码，并返回给$match[0]，按序排列组成数组
//通过array_slice将数组进行切割，然后通过join将切割后的数组的key进行连接

    $slice = join("",array_slice($match[0], $start, $length));

    if($suffix) return $slice."…";

    return $slice;

}

$str1 = "卧室字符串";

echo mb_substr($str1,0,4,'utf-8')."\n";

echo mb_strcut($str1,0,4,'utf-8')."\n";

echo substr($str1,0,4)."\n";