function __construct($username, $password, $env, $campaignid)
 {
     if (!in_array($env, array('test', 'production'))) {
         $this->error('Environment "' . htmlentities($env) . '" is not valid. Only "production" and "test" are supported.');
     }
     self::$username = $username;
     self::$password = $password;
     self::$env = $env;
     self::$campaignid = $campaignid;
 }