code: public function charAt($index)
      {
          $index = (int) $index;
  
          if ($index < 0 || $index >= $this->length()) {
              throw new \OutOfBoundsException();
          }
  
          return mb_substr($this->string, $index, 1, $this->encoding);
      }
docs: Return the character at the given position in the sequence.
  
  @param int $index The character index.
  
  @return string
  @throws \OutOfBoundsException If the index is out of bounds.