print_r(str_split('dw氛围fesf',3));

function mbStringToArray($str) {

    if(empty($str)){

       return false;

    }

    $len = mb_strlen($str);

    $array = array();

    for($i = 0; $i<$len; $i++) {

        $array[] = mb_substr($str, $i, 1);

    }

    return $array;

}

/**

  * @param str $str

  * @param int $length 截取长度

  * @param boole $byte 是否按字节分隔 false 按字符数分隔

  * @return array

*/

function _str_split($str,$length,$byte=false){

    if(mb_strwidth($str) == 1 || empty($str)){

       return $str;

    }

    if($encoding = mb_detect_encoding($str, null, true) === false ){

       return str_split($str, $length);

    }

    $utf8_str = mb_convert_encoding($str, 'utf8', $encoding);

    if($byte){

        $line = '';

        $split_arr = [];

        foreach (preg_split('//u', $utf8_str,-1,PREG_SPLIT_NO_EMPTY ) as $char) {

            $width = mb_strwidth($line.$char,'utf8');

            if($width <= $length){

                $line .= $char;

                continue;

            }

            $split_arr[] = str_pad($line, $width);

            $line = $char;

        }

        return $split_arr;

    }else{

        $str_arr = mbStringToArray($str);

        if($str_arr){

            $chunk_index = 0;

            $k_index = 0;

            $line = '';

            $chunks = [];

            foreach ($str_arr as $key=>$val){

                $line .= $val;

                $chunks[$k_index] = $line;

                if ($chunk_index++ == $length-1) {

                    $line = '';

                    $k_index++;

                    $chunk_index = 0;

                }

            }

        }

        return $chunks;

    }

}

print_r(_str_split($str,3,false));