[Wed Mar 14 10:41:39.684427 2018] [:error] [pid 1366] [client ::1:43724] PHP Catchable fatal error:  Object of class stdClass could not be converted to string in /var/www/html/test/index.php on line 44

try {

    $proxy = new SoapClient('http://toso.mitocondria.cl/index.php/api/v2_soap/?wsdl');

    $sessionId = $proxy->login((object)array('username' => 'user', 'apiKey' => 'password'));

    $result = $proxy->catalogCategoryTree((object)array('sessionId' => $sessionId->result, 'parentId' => '2'));
    var_dump($result->result);

} catch (Exception $ex) {
    echo 'Error: '.$ex->getMessage();
}

$sessionId = $proxy->login((object)array('username' => 'user', 'apiKey' => 'password'));