private function checkAppToken()
{
// check app token (if needed)
if (!isset($this->parameters['app_token'])) {
$this->parameters['app_token'] = "";
}
if (!($this->apiclients_id = array_search($this->parameters['app_token'], $this->app_tokens))) {
$this->returnError(__("missing parameter app_token"), 400, "ERROR_APP_TOKEN_PARAMETERS_MISSING");
}
}