__call equivalent for public methods

I have an API for interacting with my web app, defined by a class. Each publicly accessible method needs to have authentication done before running. Rather than putting the same line over and over in each method, I'd like to use the magic __call function. However, it will only work on private or protected methods, and mine need to be public in order to work with Zend_Json_Server.

class MY_Api
{
  public function __call($name, $arguments)
  {
    //code here that checks arguments for valid auth token and returns an error if false
  }

  public function myFunction($param1, $param2, $param3)
  {
    //do stuff when the user calls the myFunction and passes the parameters
    //this function must remain public so that Zend_Json_Server can parse it
    //but I want it intercepted by a magic method so that the authentication
    //can be checked and the system bails before it even gets to this function.
  }
}

Is it possible to hook into these public functions and possibly cancel their execution before they are called?