Implementing a very basic set using inheritance / polymorphism

EmptyNode.php

<?php

class EmptyNode implements Node {
    function add($value) {
        return new ValueNode($this, $this, $value);
    }

    function contains($value) {
        return false;
    }
}

Node.php

<?php

interface Node {
    function add($value);
    function contains($value);
}

Set.php

<?php

class Set {

    /**
     * @var Node
     */
    private $root;

    function __construct() {
        $this->root = new EmptyNode();
    }

    function add($value) {
        $this->root = $this->root->add($value);
    }

    function contains($value) {
        return $this->root->contains($value);
    }

}

ValueNode.php

<?php

class ValueNode implements Node {
    public $left;
    public $right;
    public $value;


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

    function add($value) {
        if ($value < $this->value) {
            return $this->left->add($value);
        } else if ($value > $this->value) {
            return $this->right->add($value);
        } else {
            return $this;
        }
    }

    function contains($value) {
        if ($value < $this->value) {
            return $this->left->contains($value);
        } else if ($value > $this->value) {
            return $this->right->contains($value);
        } else {
            return true;
        }
    }
}