PHP English string interception code sharing that can ensure word integrity

Directly on the code:

/**
   *  Interception of complete words 
   *
   * @param $str
   * @param $start
   * @param $length
   *
   * @return string
   */
  public static function usubstr($str, $start, $length = null)
  {
 
    //  Normal interception first 1 Pass .
    $res = substr($str, $start, $length);
    $strlen = strlen($str);
 
    /*  Then judge the head and tail 6 Is the byte complete ( Not incomplete ) */
    //  If the parameter start Is a positive number 
    if ($start >= 0) {
      //  Go forward and intercept about 6 Byte 
      $next_start = $start + $length; //  Initial position 
      $next_len = $next_start + 6 <= $strlen ? 6 : $strlen - $next_start;
      $next_segm = substr($str, $next_start, $next_len);
      //  If the 1 Bytes are not   The first byte of a complete character ,  And then intercept about 6 Byte 
      $prev_start = $start - 6 > 0 ? $start - 6 : 0;
      $prev_segm = substr($str, $prev_start, $start - $prev_start);
    } // start Is a negative number 
    else {
      //  Go forward and intercept about 6 Byte 
      $next_start = $strlen + $start + $length; //  Initial position 
      $next_len = $next_start + 6 <= $strlen ? 6 : $strlen - $next_start;
      $next_segm = substr($str, $next_start, $next_len);
 
      //  If the 1 Bytes are not   The first byte of a complete character ,  And then intercept about 6 Byte .
      $start = $strlen + $start;
      $prev_start = $start - 6 > 0 ? $start - 6 : 0;
      $prev_segm = substr($str, $prev_start, $start - $prev_start);
    }
    //  Before judgment 6 Does the byte match utf8 Rules 
    if (preg_match('@^([x80-xBF]{0,5})[xC0-xFD]?@', $next_segm, $bytes)) {
      if (!empty($bytes[1])) {
        $bytes = $bytes[1];
        $res .= $bytes;
      }
    }
    //  After judgment 6 Does the byte match utf8 Rules 
    $ord0 = ord($res[0]);
    if (128 <= $ord0 && 191 >= $ord0) {
      //  Backward interception  ,  And added to res The front of .
      if (preg_match('@[xC0-xFD][x80-xBF]{0,5}$@', $prev_segm, $bytes)) {
        if (!empty($bytes[0])) {
          $bytes = $bytes[0];
          $res = $bytes . $res;
        }
      }
    }
    if (strlen($res) < $strlen) {
      $res = $res . '...';
    }
    return $res;
  }