username_check.php

// ------------------------------------------------------------------------

/**
* Validate the username
* 
* @access	public
* @return	string
*/
public function username_check($username)
{
	if ($this->events->active_hook('users_controller/username_check'))
	{
		return $this->events->trigger('users_controller/username_check', $username);
	}
	
	$this->db->select('user_username')
				->from('users')
				->where('user_username', $username);
				
	$query = $this->db->get();
	
	if ($query->num_rows() > 0)
	{
		$this->form_validation->set_message('username_check', lang('lang_username_in_use'));
		return FALSE;
	}
	else
	{
		return TRUE;
	}
}

// EXAMPLE FROM EXTENSION FILE:

/**
 * Setup form validation.
 */
public function validation($username)
{
	// Do you vb username check:
	$in_use = TRUE;
	if ($in_use)
	{
		$this->_ci->form_validation->set_message('username_check', lang('lang_username_in_use'));
		return FALSE;
	}
	
	return TRUE;
}