Objects and functions

Here's a simplified version on what I'm trying to do.

class db_object {

function db_action() {

#stuff happens here

}

function buildnav() {

$connect = $this->$dbaction();

#more stuff happens

}

The error I'm getting (and never really encountered) is:

Fatal error: Method name must be a string in C:\wamp\www\whoson1st_01\classes.php

The $connect would be line they're referring to. Any idea what it means? If you need more details, just ask.