MySql Convert Charset: CONVERT(... USING ...) for Doctrine 2

doctrine.function.convert.using.php

<?php
namespace Entities\meta\Functions;

use Doctrine\ORM\Query\Lexer;
use Doctrine\ORM\Query\AST\Functions\FunctionNode;

/**
 * "CONVERT" "(" ArithmeticPrimary AliasResultVariable AliasResultVariable ")".
 * More info:
 * http://dev.mysql.com/doc/refman/5.0/en/cast-functions.html#function_convert
 *
 * @category    Entities
 * @package     Entities\meta\Functions
 * @author      ツ Liverbool <nukboon@gmail.com>
 * @license     MIT License
 */
class ConvertUsing extends FunctionNode
{
	public $field;
	public $using;
	public $charset;

	/**
	 * @override
	 */
	public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
	{
		return sprintf('CONVERT(%s USING %s)',
			$sqlWalker->walkArithmeticPrimary($this->field),
			//$sqlWalker->walkSimpleArithmeticExpression($this->using), // or remove USING and uncomment this
			$sqlWalker->walkSimpleArithmeticExpression($this->charset)
		);
	}

	/**
	 * @override
	 */
	public function parse(\Doctrine\ORM\Query\Parser $parser)
	{
		$parser->match(Lexer::T_IDENTIFIER);
		$parser->match(Lexer::T_OPEN_PARENTHESIS);

		$this->field   = $parser->ArithmeticPrimary();
		// adopt use bypass validate variable of parse by using AliasResultVariable ...!!
		$this->using   = $parser->AliasResultVariable();
		$this->charset = $parser->AliasResultVariable();

		$parser->match(Lexer::T_CLOSE_PARENTHESIS);
	}
}