<?php// Define a class to implement a binary treeclass Binary_Tree_Node {    // Define the variable to hold our data:    public $data;    // And a variable to hold the left and right objects:    public $left;    public $right;        // A constructor method that allows for data to be passed in    public function __construct($d = NULL) {        $this->data = $d;    }        // Traverse the tree, left to right, in pre-order, returning an array    // Preorder means that each node's value preceeds its children.    public function traversePreorder() {        // Prep some variables.        $l = array();        $r = array();        // Read in the left and right children appropriately traversed:        if ($this->left) { $l = $this->left->traversePreorder(); }        if ($this->right) { $r = $this->right->traversePreorder(); }                // Return a merged array of the current value, left, and right:        return array_merge(array($this->data), $l, $r);     }    // Traverse the tree, left to right, in postorder, returning an array    // Postorder means that each node's value follows its children.    public function traversePostorder() {        // Prep some variables.        $l = array();        $r = array();        // Read in the left and right children appropriately traversed:        if ($this->left) { $l = $this->left->traversePostorder(); }        if ($this->right) { $r = $this->right->traversePostorder(); }                // Return a merged array of the current value, left, and right:        return array_merge($l, $r, array($this->data));     }    // Traverse the tree, left to right, in-order, returning an array.    // In-order means that values are ordered as left children, then the    //  node value, then the right children.    public function traverseInorder() {        // Prep some variables.        $l = array();        $r = array();        // Read in the left and right children appropriately traversed:        if ($this->left) { $l = $this->left->traverseInorder(); }        if ($this->right) { $r = $this->right->traverseInorder(); }                // Return a merged array of the current value, left, and right:        return array_merge($l, array($this->data), $r);     }}// Let's create a binary tree that will equal the following:    3//                                                             / \      //                                                            h   9      //                                                               / \     // Create the tree:                                             6   a    $tree = new Binary_Tree_Node(3);$tree->left = new Binary_Tree_Node('h');$tree->right = new Binary_Tree_Node(9);$tree->right->left = new Binary_Tree_Node(6);$tree->right->right = new Binary_Tree_Node('a');// Now traverse this tree in all possible orders and display the results:// Pre-order: 3, h, 9, 6, aecho '<p>', implode(', ', $tree->traversePreorder()), '</p>';// Post-order: h, 9, 6, a, 3echo '<p>', implode(', ', $tree->traversePostorder()), '</p>';// In-order: h, 3, 6, 9, aecho '<p>', implode(', ', $tree->traverseInorder()), '</p>';?>