code: private function condition()
      {
          $result = $this->expression();
          if ($this->currentToken == '<') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgLT', $result, $result2);
          } elseif ($this->currentToken == '>') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgGT', $result, $result2);
          } elseif ($this->currentToken == '<=') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgLE', $result, $result2);
          } elseif ($this->currentToken == '>=') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgGE', $result, $result2);
          } elseif ($this->currentToken == '=') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgEQ', $result, $result2);
          } elseif ($this->currentToken == '<>') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgNE', $result, $result2);
          } elseif ($this->currentToken == '&') {
              $this->advance();
              $result2 = $this->expression();
              $result = $this->createTree('ptgConcat', $result, $result2);
          }
  
          return $result;
      }
docs: It parses a condition. It assumes the following rule:
  Cond -> Expr [(">" | "<") Expr].
  
  @return mixed The parsed ptg'd tree on success