PHP BFS on a Tree

bfs-tree.php

function BFS( $node, $data ) {
    if ( $node->data == $data )
        return $node;

    $node->visited = true;

    $queue = new Queue;
    $queue->enqueue($node);

    while ( !$queue->isEmpty() ) {
        $node = $queue->dequeue();

        if ( !$node->visited ) {
            if ( $node->data == $data )
                return $node;

            $node->visited = true;
        }

        if ( !is_null( $node->left ) )
            $queue->enqueue( $node->left );

        if ( !is_null( $node->right ) )
            $queue->enqueue( $node->right );
    }

}

class Queue {
    protected $first;
    protected $last;

    public function enqueue($node) {
        if ( is_null( $this->first ) ) {
            $this->first = $this->last = $node;
        } else {
            $this->last->next = $node;
            $this->last = $node;
        }
    }

    public function dequeue() {
        if ( is_null( $this->first ) )
            return $this->first;

        $node = $this->first;
        $this->first = $node->next;

        return $node;
    }

    public function isEmpty() {
        return is_null( $this->first );
    }
}

class Node {
    public $data;
    public $left;
    public $right;
    public $next;
    public $visited = false;

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