code: private function getIndentAt(Tokens $tokens, $index)
      {
          $content = '';
          $lineEnding = $this->whitespacesConfig->getLineEnding();
  
          // find line ending token
          for ($index; $index > 0; --$index) {
              if (false !== strstr($tokens[$index]->getContent(), $lineEnding)) {
                  break;
              }
          }
  
          if ($tokens[$index]->isWhitespace()) {
              $content = $tokens[$index]->getContent();
              --$index;
          }
  
          if ($tokens[$index]->isGivenKind(T_OPEN_TAG)) {
              $content = $tokens[$index]->getContent().$content;
          }
  
          if (1 === Preg::match('/\R{1}([ \t]*)$/', $content, $matches)) {
              return $matches[1];
          }
  
          return null;
      }
docs: @param int $index
  
  @return null|string