code: public static function sentence(array $words, $twoWordsConnector = null, $lastWordConnector = null, $connector = ', ')
      {
          if ($twoWordsConnector === null) {
              $twoWordsConnector = Yii::t('yii', ' and ');
          }
          if ($lastWordConnector === null) {
              $lastWordConnector = $twoWordsConnector;
          }
          switch (count($words)) {
              case 0:
                  return '';
              case 1:
                  return reset($words);
              case 2:
                  return implode($twoWordsConnector, $words);
              default:
                  return implode($connector, array_slice($words, 0, -1)) . $lastWordConnector . end($words);
          }
      }
docs: Converts a list of words into a sentence.
  
  Special treatment is done for the last few words. For example,
  
  ```php
  $words = ['Spain', 'France'];
  echo Inflector::sentence($words);
  // output: Spain and France
  
  $words = ['Spain', 'France', 'Italy'];
  echo Inflector::sentence($words);
  // output: Spain, France and Italy
  
  $words = ['Spain', 'France', 'Italy'];
  echo Inflector::sentence($words, ' & ');
  // output: Spain, France & Italy
  ```
  
  @param array $words the words to be converted into an string
  @param string $twoWordsConnector the string connecting words when there are only two
  @param string $lastWordConnector the string connecting the last two words. If this is null, it will
  take the value of `$twoWordsConnector`.
  @param string $connector the string connecting words other than those connected by
  $lastWordConnector and $twoWordsConnector
  @return string the generated sentence
  @since 2.0.1