Doctrine Greatest Function

Geatest.php

<?php

use Doctrine\ORM\Query\AST\Functions\FunctionNode;
use Doctrine\ORM\Query\AST\Node;
use Doctrine\ORM\Query\Lexer;
use Doctrine\ORM\Query\Parser;
use Doctrine\ORM\Query\SqlWalker;

class Greatest extends FunctionNode
{

    public $exprs;

    public function parse(Parser $parser)
    {
        $this->exprs = [];

        $lexer = $parser->getLexer();
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $this->exprs[] = $parser->ArithmeticPrimary();

        while (Lexer::T_COMMA === $lexer->lookahead['type']) {
            $parser->match(Lexer::T_COMMA);
            $this->exprs[] = $parser->ArithmeticPrimary();
        }

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

    public function getSql(SqlWalker $sqlWalker)
    {
        return 'GREATEST(' . join(',', array_map(function(Node $expr) use ($sqlWalker) {
            return $expr->dispatch($sqlWalker);
        }, $this->exprs)) . ')';
    }
}

Could be using in symfony using http://symfony.com/doc/current/doctrine/custom_dql_functions.html