PHPのsubstrで漢字の半分が文字化けしてしまう問題の解決策

/**
* substr の改善
* bbs.it-home.org を編集
*/
function getstr($string, $length, $encoding = 'utf-8') {
$string = trim($string);
 if($length && strlen($string) > $length) {
 //截断字符
 $wordscut = '';
 if(strto lower($encoding) == 'utf-8') {
 //utf8编码
 $n = 0;
 $tn = 0;
 $noc = 0;
 while ($n < strlen($string)) {
 $t = ord($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 >= $length) {
 ブレーク;
 }
 }
 if ($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);
 }
 ?>