code: private function makeVar($prefix)
      {
          if (!isset($this->vars[$prefix])) {
              $this->vars[$prefix] = 0;
              return '$' . $prefix;
          }
  
          return '$' . $prefix . ++$this->vars[$prefix];
      }
docs: Creates a monotonically incrementing unique variable name by prefix.
  
  @param string $prefix Variable name prefix
  
  @return string