recursive.php

<?php

class TreeNode
{
    private $place;

    private $children = [];

    public function __construct($place)
    {
        $this->place = $place;
    }

    public function add(TreeNode $node)
    {
        $this->children[] = $node;
    }

    public function getPlace()
    {
        return $this->place;
    }

    public function getChildren()
    {
        return $this->children;
    }
}

class TreeIterator implements RecursiveIterator
{
    private $nodes = array();

    public function __construct($nodes)
    {
        if (is_array($nodes)) {
            $this->nodes = $nodes;
        } elseif ($nodes instanceof TreeNode) {
            $this->nodes = [$nodes];
        } else {
            throw \InvalidArgumentException();
        }
    }

    public function current()
    {
        return current($this->nodes);
    }

    public function key()
    {
        return key($this->nodes);
    }

    public function next()
    {
        next($this->nodes);
    }

    public function rewind()
    {
        reset($this->nodes);
    }

    public function valid()
    {
        return current($this->nodes) !== false;
    }

    public function getChildren()
    {
        return new TreeIterator($this->current()->getChildren());
    }

    public function hasChildren()
    {
        return 0 < count($this->current()->getChildren());
    }
}

$one = new TreeNode(1);

$two = new TreeNode(2);
$three = new TreeNode(3);

$one->add($two);
$one->add($three);

$two->add(new TreeNode(4));
$two->add(new TreeNode(5));
$two->add(new TreeNode(6));

$it = new TreeIterator($one);

$rii = new RecursiveIteratorIterator($it, RecursiveIteratorIterator::SELF_FIRST);

foreach ($rii as $node) {
    echo "PLACE = " . $node->getPlace() . "\n";
}