How to get users (entities) for a certain field value?
Simple example for the use case I asked about:

$query = new EntityFieldQuery();
$query-&gt;entityCondition('entity_type', 'user')
  -&gt;fieldCondition('field_birthday', 'value', $birthday);

$result = $query-&gt;execute();
$users_ids = array_keys($result['user']);