I want my 'users' table in my database to contain users of all different levels (members, moderators, admins, etc). So a column in my 'users' table is role. I want to be able to check the role to see if they have permission to log in to special parts of the application. How can I do this? Here is my auth adapter so far:
protected function _getAuthAdapter($formData)
{
    $dbAdapter = Zend_Db_Table::getDefaultAdapter();
    $authAdapter  = new Zend_Auth_Adapter_DbTable($dbAdapter);
    $authAdapter->setTableName('users');
    $authAdapter->setIdentityColumn('username');
    $authAdapter->setCredentialColumn('password');
    $authAdapter->setIdentity($formData['username']);
    $authAdapter->setCredential(md5($formData['password']));
    return $authAdapter;
}