code: public function getUniqueWords($string)
      {
          $words = $this->getWords($string);
          $analysis = new FrequencyAnalysis($words);
          $words = $analysis->getKeyValuesByFrequency();
  
          return array_unique(array_keys($words));
      }
docs: Get unique words.
  
  @param string $string
  
  @return array