Doctrine Many to Many Insert

You have lot of errors (pay attention to grammar):

instead of

public $categories;public function __construct() {    $this->category = new \Doctrine\Common\Collections\ArrayCollection();}

it should be:

protected $categories;public function __construct() {    $this->categories = new \Doctrine\Common\Collections\ArrayCollection();}

instead of:

public $user;public function __construct() {    $this->user = new \Doctrine\Common\Collections\ArrayCollection();}

use

protected $users;public function __construct() {    $this->users = new \Doctrine\Common\Collections\ArrayCollection();}

Instead of

public function addCategories(Category $category = null){    $this->categories = $category;}

it must be

public function addCategory(Category $category = null){    $this->categories->add($category);}

and

public function removeCategory(Category $category){    $this->categories->removeElement($category) ;}public function setCategories($categories){    $this->categories = categories;}

Same logic on both sides. I don't know how CI works but Symfony will automatically find addSomething/removeSomething methods. Even if CI doesn't support that feature, you should still change your code as above.