BTree tagged union

btree.class.php

<?php

abstract class Tree {}

class Leaf extends Tree {
    function __construct($n) {
        $this->value = $n;
    }
}

class Node extends Tree {
    function __construct($n, Tree $left, Tree $right) {
        $this->value = $n;
        $this->left = $left;
        $this->right = $right;
    }
}