Symfony query builder get value without keys

I have a problem getting the result I want. I always get the key together with the value but I just want to get the value.

I have some code like this:

$qb
  ->select('a.id')
  ->from(Article::class, 'a')
  ...
  ->getQuery()
  ->getArrayResult();

and my result is:

Array ( [0] => Array ( [id] => 111 ) 
        [1] => Array ( [id] => 222 ) 
)

but I want it to be like:

Array ( [0] => 111 [1] => 222 )

I have tried to user ->getResult(), ->getArrayResult() and stuff like that but I never came to the right solution.

Thank you very much!