d2-bootstrap.php

<?php

$config = new \Doctrine\ORM\Configuration();
/** OTHER STUFF */
$config->addCustomNumericFunction('RAND', 'My\Custom\Doctrine2\Function\Rand');
/** CONTINUES */

Rand.php

<?php
namespace My\Custom\Doctrine2\Function;

/**
 * RandFunction ::= "RAND" "(" ")"
 */
class Rand extends FunctionNode
{

    public function parse(\Doctrine\ORM\Query\Parser $parser)
    {
        $parser->match(Lexer::T_IDENTIFIER);
        $parser->match(Lexer::T_OPEN_PARENTHESIS);
        $parser->match(Lexer::T_CLOSE_PARENTHESIS);
    }

    public function getSql(\Doctrine\ORM\Query\SqlWalker $sqlWalker)
    {
        return 'RAND()';
    }
}