function get_random_ids_from_table(&$link, $table, $where = null, $length = -1)
{
    $ids_r = mysqli_query($link, 'select id from `' . $table . '`' . ($where ? ' where ' . $where : '') . ' order by rand()' . ($length && $length > 0 ? ' limit ' . $length : ''));
    if(!$ids_r)
    {
        return array();
    }

    $count = mysqli_num_rows($ids_r);

    if(!$count)
    {
        return array();
    }

    $ids = array();

    while($id = mysqli_fetch_assoc($ids_r))
    {
        $ids[] = $id['id'];
    }
    mysqli_free_result($ids_r);

    return $ids;
}