CollabController.php

<?php

class CollabController extends AbstractController
{
    /**
     * @Route("/editmatform", name="collab_edit_mat_form", methods={"GET","POST"})
     */
    public function editmatform(Request $request)
    {
        $em = $this->getDoctrine()->getManager();
        $id_collab = $request->request->get("collab");
        $retiremat = $request->request->get("cbRetiremat");
        $addMat = $request->request->get("cbAddmat");

        /** @var \App\Entity\Collab $collab */
        $collab = $em->getRepository(Collab::class)->find($id_collab);
        $materiels = $collab->getMateriels();

        if (! $retiremat) {
            return $this->redirectToRoute('collab_edit_mat', ['id' => $id_collab]);
        }

        foreach ($materiels as $materiel) {
            if (in_array($materiel->getId(), $retiremat, false)) {
                $collab->removeMteriel($materiel);
            }
            $em->flush();
        }

        return $this->redirectToRoute('collab_edit_mat', ['id' => $id_collab]);
    }
}