My linked list

tree_to_list.php

<?
class Tree {
  function add($value) {
    if (!isset($this->value)) {
      $this->value = $value;
      return;
    }

    if ($value < $this->value) {
      if (!$this->left) $this->left = new Tree();
      $this->left->add($value);
    } else if ($value > $this->value) {
      if (!$this->right) $this->right = new Tree();
      $this->right->add($value);
    }
  }

  function toList() {
    $array = array();
    $this->toArray($array);
    $arrayLength = count($array);
    for($i = 0; $i < $arrayLength; $i++) {
      if ($i > 0) {
        $array[$i]->left = $array[$i - 1];
      }
      if ($i < $arrayLength - 1) {
        $array[$i]->right = $array[$i + 1];
      }
    }
    return $array;
  }

  private function toArray(&$array) {
    if ($this->left) {
      $this->left->toArray($array);
    }
    $array[] = $this;
    if ($this->right) {
      $this->right->toArray($array);
    }
  }
}

$tree = new Tree();
$tree->add(10);
$tree->add(5);
$tree->add(15);
$tree->add(14);
$tree->add(12);
$tree->add(13);
$tree->add(16);
$tree->add(17);
$tree->add(2);
$tree->add(8);
$tree->add(7);
$tree->add(9);

print_r($tree);

$list = $tree->toList();
foreach($list as $node) {
  echo $node->value;
  echo " (left: ";
  if ($node->left) {
    echo $node->left->value;
  } else {
    echo "none";
  }
  echo ", right: ";
  if ($node->right) {
    echo $node->right->value;
  } else {
    echo "none";
  }
  echo ")\n";
}
?>