<?

/*

 * This example would probably work best if you're using

 * an MVC framework, but it can be used standalone as well.

 *

 * This example also assumes you are using Predis, the excellent

 * PHP Redis library available here:

 * https://github.com/nrk/predis

 */

class UserNode {

    // The user's ID, probably loaded from MySQL

    private $id;

    // The redis server configuration

    private $redis_config = array(

        array('host' => 'localhost', 'port' => 6379 )

    );

    // Stores the redis connection resource so that

    // we only need to connect to Redis once

    private $redis;

    public function __construct($userID) {

        $this->id = $userID;

    }

    private function redis() {

        if (!$this->redis) {

            $this->redis = new Predis\Client($redis_config);

        }

        return $this->redis;

    }

    /*

     * Makes this user follow the user with the given ID.

     * In order to stay efficient, we need to make a two-way

     * directed graph. This means when we follow a user, we also

     * say that that user is followed by this user, making a forward

     * and backword directed graph.

     */

    public function follow($user) {

        $this->redis()->sadd("graph:user:{$this->id}:following", $user);

        $this->redis()->sadd("graph:user:$user:followed_by", $this->id);

    }

    /*

     * Makes this user unfollow the user with the given ID.

     * First we check to make sure that we are actually following

     * the user we want to unfollow, then we remove both the forward

     * and backward references.

     */

    public function unfollow($user) {

        if ($this->is_following()) {

            $this->redis()->srem("graph:user:{$this->id}:following", $user);

            $this->redis()->srem("graph:user:$user:followed_by", $this->id);

        }

    }

    /*

     * Returns an array of user ID's that this user follows.

     */

    public function following() {

        return $this->redis()->smembers("graph:user:{$this->id}:following");

    }

    /*

     * Returns an array of user ID's that this user is followed by.

     */

     public function followed_by() {

         return $this->redis()->smembers("graph:user:{$this->id}:followed_by");

     }

    /*

     * Test to see if this user is following the given user or not.

     * Returns a boolean.

     */

    public function is_following($user) {

        return $this->redis()->sismember("graph:user:{$this->id}:following", $user);

    }

    /*

     * Test to see if this user is followed by the given user.

     * Returns a boolean.

     */

    public function is_followed_by($user) {

        return $this->redis()->sismember("graph:user:{$this->id}:followed_by", $user);

    }

    /*

     * Tests to see if the relationship between this user and the given user is mutual.

     */

    public function is_mutual($user) {

        return ($this->is_following($user) && $this->is_followed_by($user));

    }

    /*

     * Returns the number of users that this user is following.

     */

    public function follow_count() {

        return $this->redis()->scard("graph:user:{$this->id}:following");

    }

    /*

     * Retuns the number of users that follow this user.

     */

    public function follower_count() {

        return $this->redis()->scard("graph:user:{$this->id}:followed_by");

    }

    /*

     * Finds all users that the given users follow in common.

     * Returns an array of user IDs

     */

    public function common_following($users) {

        $redis = $this->redis();

        $users[] = $this->id;

        $keys = array();

        foreach ($users as $user) {

            $keys[] = "graph:user:{$user}:following";

        }

        return call_user_func_array(array($redis, "sinter"), $keys);

    }

    /*

     * Finds all users that all of the given users are followed by in common.

     * Returns an array of user IDs

     */

    public function common_followed_by($users) {

        $redis = $this->redis();

        $users[] = $this->id;

        $keys = array();

        foreach ($users as $user) {

            $keys[] = "graph:user:{$user}:followed_by";

        }

        return call_user_func_array(array($redis, "sinter"), $keys);

    }

}

<?

// create two user nodes, assume for this example

// they're users with no social graph entries.

$user1 = UserNode(1);

$user2 = UserNode(2);

$user1->follows(); // array()

// add some followers

$user1->follow(2);

$user1->follow(3);

// now check the follow list

$user1->follows(); // array(2, 3)

// now we can also do:

$user2->followed_by(); // array(1)

// if we do this...

$user2->follow(3);

// then we can do this to see which people users #1 and #2 follow in common

$user1->common_following(2); // array(3)