code: public function insert($offset, $string)
      {
          $offset = (int) $offset;
          $string = static::convertString($string, $this->encoding);
  
          if ($offset < 0 || $offset >= $this->length()) {
              throw new \OutOfBoundsException();
          }
  
          $this->string = mb_substr($this->string, 0, $offset, $this->encoding) .
              $string .
              mb_substr($this->string, $offset, $this->length(), $this->encoding);
          return $this;
      }
docs: Insert a string into the sequence.
  
  @param int    $offset The offset index.
  @param string $string The sequence to insert.
  
  @return StringBuilder
  
  @throws \OutOfBoundsException If the index is out of bounds.