Binary Tree Data Structure Using PHP

tree.php

<?php
// https://www.baeldung.com/java-binary-tree
class Node {
  public $left, $right;
  public $value;
  public function __construct($value) {
    $this->value = $value;
    $this->left = null;
    $this->right = null;
  }
}

class BinaryTree {
  public $root;

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

  public function insert($value) {
        if($this->root == null) {
          $this->root = new Node($value);
        } else if($value > $this->root->data) {
          
          $this->root->left = $this->insert($value, $this->root->left);
        } else if($value < $root->data) {
          $this->root->right = $this->insert($value, $this->root->right);
        }
    }

  public function  addNode($value) {
    $this->root = $this->addNodeRecursively($this->root, $value);
  }
  private function addNodeRecursively(?Node $current, int $value) {
    if ($current == null) {
        return new Node($value);
    }
    if ($value < $current->value) {
        $current->left = $this->addNodeRecursively($current->left, $value);
    } else if ($value > $current->value) {
        $current->right = $this->addNodeRecursively($current->right, $value);
    } else {
        // value already exists
        return $current;
    }

    return $current;
}

// perform an in-order traversal of the current node
    public function traverseInOrder($root) {
        if($root == NULL) {return; }
      
        $this->traverseInOrder($root->left);
        echo $root->value."\n";
        $this->traverseInOrder($root->right);
        
    }
}

$tree = new BinaryTree();
$tree->addNode(9);
$tree->addNode(6);
$tree->addNode(6);

$tree->traverseInOrder($tree->root);