How to get a one-dimensional scalar array as a doctrine dql query result?

A better solution is to use PDO:FETCH_COLUMN . To do so you need a custom hydrator:

//MyProject/Hydrators/ColumnHydrator.php
namespace DoctrineExtensions\Hydrators\Mysql;

use Doctrine\ORM\Internal\Hydration\AbstractHydrator, PDO;

class ColumnHydrator extends AbstractHydrator
{
    protected function hydrateAllData()
    {
        return $this->_stmt->fetchAll(PDO::FETCH_COLUMN);
    }
}

Add it to Doctrine:

$em->getConfiguration()->addCustomHydrationMode('COLUMN_HYDRATOR', 'MyProject\Hydrators\ColumnHydrator');

And you can use it like this:

$em->createQuery("SELECT a.id FROM Auction a")->getResult("COLUMN_HYDRATOR");

More info: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/dql-doctrine-query-language.html#custom-hydration-modes

PHP < 5.5

You can use array_map, and since you only have on item per array, you can elegantly use 'current' as callback, instead of writing a closure.

$result = $em->createQuery("SELECT a.id FROM Auction a")->getScalarResult();
$ids = array_map('current', $result);

See Petr Sobotka's answer below for additional info regarding memory usage.

PHP >= 5.5

As jcbwlkr's answered below, the recommended way it to use array_column.

As of PHP 5.5 you can use array_column to solve this

$result = $em->createQuery("SELECT a.id FROM Auction a")->getScalarResult();
$ids = array_column($result, "id");

Ascarius' answer is elegant, but beware of memory usage! array_map() creates a copy of passed array and effectively doubles memory usage. If you work with hundreds of thousands of array items this can become an issue. Since PHP 5.4 call-time pass by reference has been removed so you cannot do

// note the ampersand
$ids = array_map('current', &$result);

In that case you can go with obvious

$ids = array();
foreach($result as $item) {
  $ids[] = $item['id'];
}