I base my templates on css, one element id in templates changes depending on sections of the site. I need custom function to return needed value of that id depending on where the user is. I'm aware of InSection() control but I want to do this in page controller to get cleaner code in template.

I've tried with function inside page_controller class but problem is that i can't access InSection() function from there. It seems to me that some part of the puzzle regarding page page_controller relation i don't understand right

Any hint regarding that or help with implementing required function will be appreciated. :)

Controllers have fall back functions to call the same method (or variable) on the underlying dataobject, so it should call InSection() fine.

e.g In your controller, you could go:
$this->InSection("home");

is the same as calling

$page = $this->data()
return $page->InSection("home");

Perhaps your not in the controller, or your doing something else?

For more information about controllers, have a look at this:
http://doc.silverstripe.com/doku.php?id=designer-guide&s=controllers

Thank you. I knew it must be something simple.