$changes = [
  "groups" => [
    "1" => [
      "workers" => [
        "1" => "manager",
        "2" => "remove",
        "3" => "member",
      ],
    ],
    "2" => [
      "workers" => [
        "2" => "remove",
      ],
    ],
  ],
];

$postfields = array(
    array('json', json_encode($changes)),
);
$out = $cerb->put($base_url . 'groups/123.json', $postfields);