code: public static function getMutation(array &$tokens, $index)
      {
          $tokenCount = count($tokens);
          for ($i=$index+1; $i < $tokenCount; $i++) {
              if (is_array($tokens[$i]) && $tokens[$i][0] == T_WHITESPACE) {
                  continue;
              } elseif (is_array($tokens[$i]) && $tokens[$i][0] == T_VARIABLE && $tokens[$i][1] == '$this') {
                  $tokens[$i] = [
                      T_STRING,
                      'null'
                  ];
                  break;
              }
          }
      }
docs: Replace (return $this;) with (return null;)
  
  @param array $tokens
  @param int $index
  @return array