Build Tree objects and print it from flatten array with parentId keys.

testBinaryTree.php

<?php

class BinaryTree
{
    private $items = [];
    private $tree;

    public function __construct($items)
    {
        foreach ($items as $item) {
            $item = (object)$item;
            $this->items[$item->id] = $item;
        }

        usort($this->items, function($a, $b) {
            return $a->parentId > $b->parentId;
        });
    }

    public static function fromArray(array $items)
    {
        return new self($items);
    }

    public function build()
    {
        $tree = $this->buildTree($this->items);
        $this->tree = reset($tree);
        return $this;
    }

    private function buildTree(array &$nodes, $parentId = 0) {
        $tree = [];

        foreach ($nodes as $node) {
            if ($node->parentId == $parentId) {
                $children = $this->buildTree($nodes, $node->id);
                if ($children) {
                    $leaves = array_values($children);
                    isset($leaves[0]) && $node->left = $leaves[0];
                    isset($leaves[1]) && $node->right = $leaves[1];
                }
                $tree[$node->id] = $node;
                unset($nodes[$node->id]);
            }
        }

        return $tree;
    }

    public function dump()
    {
        print_r($this->tree);
    }

    public function render()
    {
        return $this->renderTree($this->tree);
    }

    public function renderTree($node, $depth = 0)
    {
        if ($depth >= 5) {
            return '';
        }

        $html = "<div>";
        $html .= "<h2><span>{$node->id}</span></h2>";
        
        if (!empty($node->left)) {
            $html .= "<div class='left'>" . $this->renderTree($node->left, $depth+1) . "</div>";
        }

        if (!empty($node->right)) {
            $html .= "<div class='right'>" . $this->renderTree($node->right, $depth+1) . "</div>";
        }

        $html .= "</div>";

        return $html;
    }
}

$items = [
    ['id' => 1, 'parentId' => 0],

    ['id' => 2, 'parentId' => 1],
    ['id' => 5, 'parentId' => 1],

    ['id' => 4, 'parentId' => 2],
    ['id' => 3, 'parentId' => 2],
    ['id' => 6, 'parentId' => 3],
    ['id' => 7, 'parentId' => 6],
    ['id' => 8, 'parentId' => 7],
    ['id' => 9, 'parentId' => 8],

];

$tree = BinaryTree::fromArray($items);
//$tree->build()->dump();
$html = $tree->build()->render();
?>

<style>
    body {text-align: center; }
    h2 { margin: 5px 0;  }
    h2 span { background-color: #5b8039; padding: 5px; }
    .left, .right { width: 50%; vertical-align:top; }
    .left { float: left; }
    .right { float: right; }
</style>

<?= $html ?>