Doctrine 2 ManyToMany - the correct way

User.php

<?php

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * @ORM\Entity()
 * @ORM\Table(name="user")
 */
class User
{
    /**
     * @var int|null
     * @ORM\Id()
     * @ORM\GeneratedValue(strategy="AUTO")
     * @ORM\Column(type="integer", name="id")
     */
    protected $id;

    /**
     * @var \Doctrine\Common\Collections\Collection|UserGroup[]
     *
     * @ORM\ManyToMany(targetEntity="UserGroup", inversedBy="users")
     * @ORM\JoinTable(
     *  name="user_usergroup",
     *  joinColumns={
     *      @ORM\JoinColumn(name="user_id", referencedColumnName="id")
     *  },
     *  inverseJoinColumns={
     *      @ORM\JoinColumn(name="usergroup_id", referencedColumnName="id")
     *  }
     * )
     */
    protected $userGroups;

    /**
     * Default constructor, initializes collections
     */
    public function __construct()
    {
        $this->userGroups = new ArrayCollection();
    }

    /**
     * @param UserGroup $userGroup
     */
    public function addUserGroup(UserGroup $userGroup)
    {
        if ($this->userGroups->contains($userGroup)) {
            return;
        }

        $this->userGroups->add($userGroup);
        $userGroup->addUser($this);
    }

    /**
     * @param UserGroup $userGroup
     */
    public function removeUserGroup(UserGroup $userGroup)
    {
        if (!$this->userGroups->contains($userGroup)) {
            return;
        }

        $this->userGroups->removeElement($userGroup);
        $userGroup->removeUser($this);
    }
}

UserGroup.php

<?php

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * @ORM\Entity()
 * @ORM\Table(name="usergroup")
 */
class UserGroup
{
    /**
     * @var int|null
     * @ORM\Id()
     * @ORM\GeneratedValue(strategy="AUTO")
     * @ORM\Column(type="integer", name="id")
     */
    protected $id;

    /**
     * @var \Doctrine\Common\Collections\Collection|User[]
     *
     * @ORM\ManyToMany(targetEntity="User", mappedBy="userGroups")
     */
    protected $users;

    /**
     * Default constructor, initializes collections
     */
    public function __construct()
    {
        $this->users = new ArrayCollection();
    }

    /**
     * @param User $user
     */
    public function addUser(User $user)
    {
        if ($this->users->contains($user)) {
            return;
        }

        $this->users->add($user);
        $user->addUserGroup($this);
    }

    /**
     * @param User $user
     */
    public function removeUser(User $user)
    {
        if (!$this->users->contains($user)) {
            return;
        }

        $this->users->removeElement($user);
        $user->removeUserGroup($this);
    }
}

@haydenk a specific return value is outside the scope for this gist, which is simply demonstrating how to handle the bi-directional object associations.
Ultimately the return values would depend on your desired behavior. You could also return $this; to allow for method chaining,
or return $this->someOtherMthod($object); to perform additional business logic and it's return value. Really is up to you.

@haydenk I also usually do returns as soon as I can so the processor doesn't have to read the entire function before continuing.
Also, not doing that you often see unneeded extra indentation in your whole function.

How would you query for a user that is in multiple groups?
eg.
User1 is in Group1 and Group2,
User2 is in Group1,
User3 is in Group2

The query should return only User1.

@werdck I would look into Doctrine specific methods. In that scenario the ORM would load the data for you and groups would be a relationship you can search through the array or collection.

https://www.doctrine-project.org/projects/doctrine-collections/en/stable/index.html