// a Foo Object
print_r($foo);

Foo Object
(
    [name] => Jon
    [email] => jon@snow.fr
    [_id] => MongoId Object
        (
            [$id] => 53c6bed12ce99716008b456f
        )

)

// Access to _id
var_dump($foo->_id);

NULL

// Check $foo->_id is public
$refl = new \ReflectionObject($merchant);
$prop = $refl->getProperty('_id');
var_dump($prop->isPublic());

bool(true)