$user_list = [];
while ($row = mysqli_fetch_assoc($res)) {
$k++;
$a = $row['userid'];
$b = $row['description'];
$c = $row['username'];
$d = $row['category'];
$user_list[] = explode(' ', trim($row['username']));
или
$user_list[ $row['userid'] ] = explode(' ', trim($row['username']));
}
var_dump($user_list);