concat tree hierarchie in a recursive PHP function - php

Question

I have a user table with hierachical users. So users can have a parent user. I am trying to return an array of all child user ids of a certain user.
My function returns "null". What's wrong?
public function userDownline($userid, $result = array()) {
$dbconn = $this->DBase();
$children = $dbconn->GetAll('SELECT id FROM users WHERE parent=' . (int)$userid);
if(count($children) > 0) {
foreach($children As $k=>$v) {
if(!in_array($v['id'], $result)) $result[] = $v['id'];
$this->userDownline($v['id'], $result);
}
} else {
return $result;
}
}

Answer 1

Of course it will return null, because you are in block if(count($children)) and there is no return from this.
I think you have to do something like this:
<?php
public function userDownline($userid, &$result = array())
{
$dbconn = $this->DBase();
$children = $dbconn->GetAll('SELECT id FROM users WHERE parent=' . (int)$userid);
if (count($children) > 0) {
foreach ($children As $k => $v) {
if (!in_array($v['id'], $result)) $result[] = $v['id'];
$this->userDownline($v['id'], $result);
}
}
return $result;
}
I added reference in function signature and move return out of the conditional block.
But this is really totaly inefficient way and dangerous(because of - out of memory, too many nesting level and other exceptions).
There are 2 better ways:
Use https://neo4j.com/ - Graph Database - best option for your task.
If you still wanna use only Sql DB - read about Nested set Model http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/