private variable append not persistent

[eluser]OttoVanzig[/eluser]
hi everybody,
I don't find the way to modify a private variable value and retrieve it in another method;
My issue :

Code:

class .. extend CI_C...
private $var;

function ..() {
  parent::__construct();
//here I load $var
$this->var[] = 'ok';

function modify()
//here I append $var
$this->var[] = 'more';

function another()   //with AJAX call
//if I check $var value, the value 'more' is not set, only 'ok';

So is there a way to append $var outside from the constructor and have it updated in the whole class ?
thanks for your help