Why does a primitive turn into an object?

Question

PHP

Data is coming:

print_r($obj);

SimpleXMLElement Object
(
    [id] => 1082930
    [state] => REGISTERED
)

I need to take id from $obj and make a new request with parameters:

$params = [
    'sector' => SECTOR_ID,
    'id' => $obj->id,
    'client_ref' => $user->client_ref,
  ];

I do:

print_r($params);

Array
(
    [sector] => 2721
    [id] => SimpleXMLElement Object
        (
            [0] => 1082930
        )

    [client_ref] => test
)

I do:

echo $obj->id;

# результат
1082930

Why do I get an object in the first case?