php substr截断中文半个汉字乱码问题的解决方法

/**
* 改进的substr
* edit bbs.it-home.org
*/
function getstr($string, $length, $encoding  = 'utf-8') {
$string = trim($string);
    if($length && strlen($string) > $length) {
            //截断字符
            $wordscut = '';
            if(strtolower($encoding) == 'utf-8') {
                //utf8编码
                $n = 0;
                $tn = 0;
                $noc = 0;
                while ($n                     $t = ord($string[$n]);
                    if($t == 9 || $t == 10 || (32                         $tn = 1;
                        $n++;
                        $noc++;
                    } elseif(194                         $tn = 2;
                        $n += 2;
                        $noc += 2;
                    } elseif(224                         $tn = 3;
                        $n += 3;
                        $noc += 2;
                    } elseif(240                         $tn = 4;
                        $n += 4;
                        $noc += 2;
                    } elseif(248                         $tn = 5;
                        $n += 5;
                        $noc += 2;
                    } elseif($t == 252 || $t == 253) {
                        $tn = 6;
                        $n += 6;
                        $noc += 2;
                    } else {
                        $n++;
                    }
                    if ($noc >= $length) {
                        break;
                    }
                }
                if ($noc > $length) {
                    $n -= $tn;
                }
                $wordscut = substr($string, 0, $n);
            } else {
                for($i = 0; $i                     if(ord($string[$i]) > 127) {
                        $wordscut .= $string[$i].$string[$i + 1];
                        $i++;
                    } else {
                        $wordscut .= $string[$i];
                    }
                }
            }
            $string = $wordscut;
        }
        return trim($string);
    }
  ?>