Folding Category Menu

I am designing a catalogue for a company that has 50 or so subcategories inside around 10 main categories. All categories and sub categories has to be displayed in the left hand side. I have managed to display it in a tree fashion but I need to when click on the main category to open below the subcategories and when clicked again to close, like a folding menu. Is it possible, can somebody help me?

Is is possible, but would require some PHP programming work.

There are 3 ways to do that:

1) Use server-side session object storing the tree, including the tree state, along with an appropriate viewer logic.
    (See e.g. http://www.histpop.org)
2) Use client-side Javascripting in the template which again stores the tree and tree state.
3) Use a CSS-driven tree (see e.g. http://www.vcsproducts.com.au)

... and if you are feeling really adventurous?

You could extend the server side idea by using AJAX. OpenCart already uses JQuery.