code: protected function isStringLiteral(&$length, &$content)
      {
          if ($this->substring(1) !== "'") {
              return false;
          }
  
          $offset = $this->offset;
          while (true) {
              $offset = $this->indexAfter("'", $offset + 1);
              if ($this->substring(1, true, $offset) !== "'") {
                  break;
              }
          }
          $length = $offset - $this->offset;
          $content = strtr($this->substring($length - 2, true, $this->offset + 1), ["''" => "'"]);
          return true;
      }
docs: {@inheritdoc}