Extending MDB2

[xconspirisist]

I'm working on trying to extend MDB2 in order to add a few proprietary functions that essentially I want to be accessed by $db.

//
// setup PEAR MDB2
//

require_once 'MDB2.php';
require_once 'MDB2/Driver/mysqli.php';

$params = array(

);

class NJTrackerDB extends MDB2 {
	var $vTablePrefix = null;
	var $vMembersTable = null;

public function tablePrefix($s = null) {
	if (isset($s)) {
		return $this->vTablePrefix;
	} else {
		$this->vTablePrefix = $s;
	}
}

public function membersTable() {
	global $db_membersTable;

	$this->vMembersTable = $db_membersTable;
	return $this->db_membersTable;

}
}

$dsn = "mysqli://$db_username:$db_password@$db_host/$db_database";
$db =& NJTrackerDB::factory($dsn, $params);
$db->connect($dsn); 

echo $db->tablePrefix();

$sql = 'SELECT * FROM `items`';
$result = $db->query($sql);

while ($row = $result->fetchRow()) {
	print_r($row);
}

At the moment, I'm getting errors about tablePrefix() not being defined, where connect does work. Can anyone suggest the proper way to do this? Thanks.