php mysql选择组织图/多级结构

I have build my user table like this:

Now I would like to select the 3 blue rows. I have build the following function:

function display_children($parent, $array = array()) {
  global $db;

  $stmt = $db->prepare("SELECT id AS man, parent_id FROM user WHERE parent_id= ?");
  $stmt->execute(array($parent));
  while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    display_children($row['man'], $array);
    $array[] = array("man" => $row['man'], "parent_id" => $row['parent_id']);
    echo $row['man']."
"; //for debug
  }
  return $array;
}
print_r(display_children(50001));

My Output is:

50002
50004
50003
Array
(
    [0] => Array
        (
            [man] => 50002
            [parent_id] => 50001
        )

    [1] => Array
        (
            [man] => 50003
            [parent_id] => 50001
        )

)

The first three lines are correct, but the array is missing one. The problem is, that the first parent_id has two rows. But the array is empty at the beginning. Is there a solution for my query?

the solution was to change the following line from

display_children($row['man'], $array);

to

$array = display_children($row['man'], $array);