/**

     * 截取中文字符串函数

     * @param $str    需要截取的字串

     * @param $start    开始截取的位置

     * @param $length    截取的长度

     * @return  此函数返回的是一个数组，因此要配合join函数来显示字符串:join('',subString_UTF8($str, $start, $lenth));

     */

    public function subString_UTF8($str, $start, $lenth){

        $len = strlen($str);

        $r = array();

        $n = 0;

        $m = 0;

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

            $x = substr($str, $i, 1);

            $a  = base_convert(ord($x), 10, 2);

            $a = substr('00000000'.$a, -8);

            if ($n < $start){

                if (substr($a, 0, 1) == 0) {

                }elseif (substr($a, 0, 3) == 110) {

                    $i += 1;

                }elseif (substr($a, 0, 4) == 1110) {

                    $i += 2;

                }

                $n++;

            }else{

                if (substr($a, 0, 1) == 0) {

                    $r[ ] = substr($str, $i, 1);

                }elseif (substr($a, 0, 3) == 110) {

                    $r[ ] = substr($str, $i, 2);

                    $i += 1;

                }elseif (substr($a, 0, 4) == 1110) {

                    $r[ ] = substr($str, $i, 3);

                    $i += 2;

                }else{

                    $r[ ] = '';

                }

                if (++$m >= $lenth){

                    break;

                }

            }

        }

        return $r;

    } // End subString_UTF8;