how check method exist

[eluser]minhbu[/eluser]
No
i have a controller demo
class Demo extends Controller{
function demo()
{
parent::Controller();
}
function index()
{
$id=$this->uri->segment(3);
if(method_exists($this,$id))
$this->$id;
else
echo "Your method haven`t got";
}
function a()
{
echo "Hello";
}
}
I have URI http://localhost/demo/index/a
but i have a error

A PHP Error was encountered
Severity: Notice

Message: Undefined property: Demo::$id

Filename: controllers/demo.php

Line Number: 20