Example of a ManyToMany Relationship in Doctrine.

Doctrine-ManyToMany.php

<?php
/** @Entity */
class User
{
    /** @Id @GeneratedValue @Column(type="string") */
    private $id;

    /**
     * Bidirectional - Many users have Many favorite comments (OWNING SIDE)
     *
     * @ManyToMany(targetEntity="Comment", inversedBy="userFavorites")
     */
    private $favorites;

    public function getFavoriteComments() {
        return $this->favorites->toArray();
    }

    public function addFavorite(Comment $comment) {
        $this->favorites->add($comment);
        $comment->addUserFavorite($this);
    }

    public function removeFavorite(Comment $comment) {
        $this->favorites->removeElement($comment);
        $comment->removeUserFavorite($this);
    }

}

/** @Entity */
class Comment
{
    /** @Id @GeneratedValue @Column(type="string") */
    private $id;

    /**
     * Bidirectional - Many comments are favorited by many users (INVERSE SIDE)
     *
     * @ManyToMany(targetEntity="User", mappedBy="favorites")
     */
    private $userFavorites;

    public function getUserFavorites() {
        return $this->userFavorites->toArray();
    }

    public function addUserFavorite(User $user) {
        $this->userFavorites[] = $user;
    }

    public function removeUserFavorite(User $user) {
        $this->userFavorites->removeElement($user);
    }
}

/*
You will notice that addUserFavorite and removeUserFavorite do not call
addFavorite and removeFavorite, thus the bidirectional association is
strictly-speaking still incomplete. However if you would naively add the
addFavorite in addUserFavorite, you end up with an infinite loop, so more work
is needed. As you can see, proper bidirectional association management in plain
OOP is a non-trivial task and encapsulating all the details inside the classes
can be challenging.
 */