public function login($username, $password, $api_key = '')
 {
     $args = array('user' => $username, 'password' => $password);
     if ($api_key) {
         //The api_key variable is only required for production accounts
         //For the sandbox this is not necessary.
         $args['api_key'] = $api_key;
     }
     $response = array();
     $response = $this->call('login', $args);
     if ($response['status_attr']['code'] != 'ok') {
         if (self::$_debug_level >= 1) {
             print "API->login = FALSE\n";
         }
         die("ERROR: Login FAILED!\n");
     } else {
         if (self::$_debug_level >= 1) {
             print "API->login = TRUE\n";
         }
         self::$_auth = array('name' => $username, 'password' => $password);
         return true;
     }
 }