PHP interception string function, PHP custom string interception method

/**
* 方法库-截取字符串
* @param string  $string 字符串
* @param int$length 字符长度
* @param string  $dot 截取后是否添加...
* @param string  $charset编码
* @return string
*/
public function cutstr($string, $length, $dot = ' ...', $charset = 'utf-8') {
   if (strlen($string) <= $length) {
  return $string;
   }
   $string = str_replace(array('&', '"', '<', '>'), array('&', '"', '<', '>'), $string);
   $strcut = '';
   if (strtolower($charset) == 'utf-8') {
  $n = $tn = $noc = 0;
  while ($n < strlen($string)) {
 $t = ord($string[$n]); //ASCII?
 if($t == 9 || $t == 10 || (32 <= $t && $t <= 126)) {
$tn = 1; $n++; $noc++;
 } elseif (194 <= $t && $t <= 223) {
$tn = 2; $n += 2; $noc += 2;
 } elseif (224 <= $t && $t < 239) {
$tn = 3; $n += 3; $noc += 2;
 } elseif (240 <= $t && $t <= 247) {
$tn = 4; $n += 4; $noc += 2;
 } elseif (248 <= $t && $t <= 251) {
$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;
  }
  $strcut = substr($string, 0, $n);
   } else {
  for ($i = 0; $i < $length; $i++) {
 $strcut .= ord($string[$i]) > 127 ? $string[$i].$string[++$i] : $string[$i];
  }
   }
   $strcut = str_replace(array('&', '"', '<', '>'), array('&', '"', '<', '>'), $strcut);
   return $strcut.$dot;
}

例2，php检测字符串是否存在：

/**
 * 方法库-字符串是否存在
 * @param string $str :字符或字符串
 * @param string $string :字符串
 * @return string 例子: $str='34' $string='1234' 返回 TRUE
 */
public function is_str_exist($str, $string) {
   $string = (string) $string;
   $str = (string) $str;
   return strstr($string,$str)===false ? false : true;
}