codeigniter flatten array of query result

I’m using a query in Codeigniter to return the ids of all the rows that belong to a user.

$this->db->select('id');
$this->db->where('user_id', 99);
$query = $this->db->get('my_table');
return $query->result_array();

This returns

Array ( [0] => Array ( [id] => 8 ) [1] => Array ( [id] => 7 ) [2] => Array ( [id] => 6 ) )

Is it possible to return a flat array like

Array ( [0] => 8 [1] => 6 [2] => 7 )

?

Answer

The CodeIgniter documentation (most particularly, query results) doesn’t list anything that will produce a flat array result (like the PDO::FETCH_COLUMN mode provided for PDOStatement::fetchAll). As such, array results will need to be flattened in your code, such as with a foreach loop:

$flat = array();
foreach($query->result_array() as $row) {
    $flat[] = $row['id'];
}

or with the special purpose array_column:

array_column($query->result_array(), 'id');

or with the general purpose array_reduce:

array_reduce(
    $results, 
    fn ($accum, $row) => array_merge($accum, [$row['id']]),
    []);

If you’re to lazy to add few lines each time you select one column, you would need to tweak CodeIgniter. Like adding some option or returning flattened array when single column is selected. I would suggest adding an option